Visualizing Chromatographic Separation of Metal Ions on a Surface

Dec 6, 2010 - Zhengbo Chen , Chenmeng Zhang , Qinggang Gao , Guo Wang , Lulu Tan ... Li Deng , Xiangyuan Ouyang , Jianyu Jin , Cheng Ma , Ying Jiang ...
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Visualizing Chromatographic Separation of Metal Ions on a Surface-Enhanced Raman Active Medium Seung Joon Lee and Martin Moskovits* Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States ABSTRACT Metal ion carboxylato complexes possess ion-specific carboxylate Raman bands. Using this attribute we follow the chromatographic separation of a microliter aliquot of an initially equimolar solution of Pb2+ and Hg2+ using the surface-enhanced Raman spectroscopy spectra of their carboxylato complexes as unique identifiers. A glass capillary whose interior is lined with a dense layer of gold nanoparticles treated with 4-mercaptobenzoic acid simultaneously acts as a separation medium and an efficient SERS reporter of the step-by-step separation process. The observed adsorption-desorption equilibrium along the capillary is shown to conform with theory. Although Hg2+ complexes with COO- much more strongly than Pb2+, it is the Pb2+ that survives the separation process as the sole surface species. We show that this is because so much mercury is taken out of solution during early separation steps that the surface equilibrium is ultimately driven toward adsorbed Pb2+. KEYWORDS Surface-enhanced Raman spectroscopy, chromatography, separation, metal carboxylates

A

By tethering MBA-functionalized AuNPs on solid substrates such as capillaries, microchannels, or plates, one can create a SERS-active chromatographic separation medium that can identify the relative contributions of metal ions on account of the unique SERS spectrum of carboxylate when it is coordinated to specific ions.16 This also offers a particularly simple method for following the separation process as it occurs. Several existing reports describe the application of SERS to separation science. SERS-active substrates, such as silverdoped sol-gel materials and e-beam lithographically patterned Ag and Au nanostructures, were incorporated directly into the architecture of a capillary column or microfluidic channel.17,18 SERS has also been used as the spectroscopic detection technique following separation by gas/liquid-chromatography, thin-layer chromatography, flow injection analysis, and capillary-electrophoresis.19 In this report, we illustrate the general use of SERS as a molecule-specific probe for the progress of the chromatographic separation of microliter aliquots of an initially equimolar solution of Pb2+ and Hg2+ on a SERS active separation medium using the SERS spectra of their carboxylato-complexes as unique identifiers. The separation medium consists of a dense layer of gold nanoparticles functionalized with MBA bound to the interior of a siloxylated glass capillary. This allows the local surface composition of the two analytes to be determined along the capillary. The observed adsorption-desorption equilibrium is quantitatively explained in terms of a simple adsorptiondesorption theory. The architecture of the SERS substrate used to separate, detect, and identify metal cations is summarized in Figure 1a. Glass capillaries (i.d. 0.8-1.1 mm × 100 mm, Kimble)

lthough surface-enhanced Raman spectroscopy (SERS)1-4 is an exceptionally sensitive spectroanalytical technique, its reliance on metals such as silver and gold diminished its specificity on account of the low chemical reactivity of those metals toward most functional groups. As a result, the development of functionalization strategies to render the SERS substrate, and ideally the most enhancing regions of the substrates (the so-called hot spots), more specific to desired molecular targets has become a major research goal. This has been accomplished in a variety of ways including antibodyantigen pairs,5 biotin-avidin linkages,6,7 and aptamers.8,9 Certain ligands, such as carboxylate are known to show IR and Raman spectroscopic shifts specific to the metal ion they bind to, especially when chelating.10-12 Functionalizing a silver or gold SERS substrate with a carboxylate-bearing bifunctional adsorbate that is also a strong SERS reporter, such as 4-mercaptobenzoic acid (MBA), which binds to the metal through its mercapto group making the carboxylate available as a coordination ligand, seems like a good strategy both to immobilize and identify metal ions (or other species) by the spectroscopic modifications they produce in the SERS spectrum of the reporter. SERS spectra of various ligands capable of chelating metal ions, such as Eriochrome Black T,13 trimercaptotriazine,14 and dithiocarbamate,15 have previously been reported. However, using SERS to determine the relative contributions of multiple metal ions in a mixture has not been reported before.

* Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected]. Received for review: 9/3/2010 Published on Web: 12/06/2010 © 2011 American Chemical Society

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FIGURE 2. SERS spectra obtained by passing pure water (a) and 1 mM aqueous solutions of one of the following metal ions: (b) Ag+, (c) Cu2+, (d) Fe2+, (e) Hg2+, (f) Pb2+, (g) Zn2+, and (h) Cd2+ through the capillary functionalized and processes as described in the text. The spectra are all of MBA, which only show significant changes for the bands associated with the COO- moiety. Intensities were normalized to the band at 1584 cm-1.

produced reproducible SERS spectra of MBA at all locations along the capillary when interrogated by the focused laser whose focal area had a diameter ∼1.5 µm (excitation wavelength, 632.8 nm; power, 4.25 mW, integration time, 3 s; number of counts per second in the strong 1584 cm-1 band, 10800 ((4000) counts/s). Moreover, the relative SERS band intensities in the spectra remained constant over the longest measuring times used (Supporting Information). The resulting capillary was connected to a syringe by Teflon tubing and mounted on a microscope glass slide with double sided-tape for the Raman measurement. Feducial marks were drawn at 5 mm intervals on the microscope slide surface to locate the position of the moving sample solution. Raman measurements were carried out on a LabRam Aramis system (Horiba Jobin Yvon) equipped with 600 grooves/mm holographic gratings and Peltier-cooled CCD. The 632.8 nm line of a Melles Griot HeNe laser was used to excite the spectra which were collected in a backscattering geometry using a confocal Raman microscope (high stability BX40) equipped with Olympus objective (LMPlanFI 50x/ 0.50). The laser was focused on the inner wall surface of the capillary. A first set of measurements were carried out by loading individual treated capillaries with a sample of 1 mM aqueous metal ion solution of Cd2+, Pb2+, Pd2+, Hg2+, Zn2+, Fe2+, Cu2+, or Ag+. For all but silver, the solutions were prepared by dissolving the metal chloride in pure water (resistivity of 18 MΩ·cm). For silver, the solution was prepared from the nitrate. Salt samples were introduced into the capillary with a syringe (3 mL). A reference SERS spectrum was also recorded with water injected into the capillary. Typical SERS spectra obtained from these samples are shown in Figure 2. They correspond closely to previously reported SERS spectra of MBA.24-26 For the purposes of this study, we focus on the characteristic Raman bands associated with the carboxyl or car-

FIGURE 1. (a) Schematic of the metal-ion separation/detection system (not to scale). The interior of the capillary is functionalized with APS to immobilize the Au nanoparticles. The capillary’s interior is then covered with a dense layer of Au nanoparticles onto which MBA is adsorbed. MBA acts simultaneously as a metal-ion chelating agent and a SERS reporter. (b) Photographs of the a few capillaries processes as per (a) together with that of an untreated.

were filled with 2% APS (99%, Aldrich)/toluene solution using a micropipette and allowed to stand for 1 hr. The capillaries were then washed with toluene, dried, and cured at ∼100 °C for 1 hr to create a stable APS layer on the capillary’s interior surface.20 The APS-functionalized capillaries were then filled with a solution of colloidal gold prepared by citrate reduction21 again using a micropipette. After an hour, the Au colloid was washed out with water and ethanol and dried in flowing nitrogen. Loose AuNPs were easily washed out of the capillary so that all of the SERS signals originate from surface-anchored AuNPs. The high density of coupled nanoparticles ensures an abundance of SERS-active “hot spots” in the laser’s focal spot (Supporting Information).30,22 After attachment to the interior of the capillary, the AuNPs were further functionalized with MBA by dipping the capillary in 1 mM ethanolic MBA solution for 1 h. The thiol (-SH) functional groups in MBA form thermodynamically more stable bonds to the gold surface than (-COOH) does, which adsorbs as carboxylate.23 MBA will therefore bind to the gold surface preferentially through the thiol, leaving the free carboxylate as a potential ligand for metal (or other) cations. The capillaries treated in this fashion were dark violet in color, as expected for colloidal Au (Figure 1b), and (because the nanoparticle density was so great) © 2011 American Chemical Society

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increased in the presence of metal ions in comparison to what is observed with water injected into the capillary. The intensity increase is understandable. With the much more polarizable metal ion coordinated to the carboxylate, the polarizability changes accompanying the vibration should be significantly increased. Relying on conventional interpretation,10 a decrease in the value of νsym(COO-) implies a unidentate mode of bonding. However, the data quoted in the literature are based on species in solution, while our data refers to an organic acid strongly adsorbed to Au. It is known that adsorbing an acid to metal can strongly perturb its acid/ base properties.27 Hence, despite the shift to lower frequency values, the carboxylate may well be chelating the ions in a bidentate fashion and the unusual shifts are because chelation modifies both the structure of the carboxylate and the metal-adsorbate bond, the net result of which is the observed decrease in the frequency of the νsym(COO-) band on coordination with most of the metal ions studied. The frequency and intensity changes are sufficiently specific that when only a single metal ion is present (from among Cd2+, Pb2+, Pd2+, Hg2+, Zn2+, Fe2+, Cu2+, and Ag+) one can unequivocally determine its identity (Figure 3). The values of the spectroscopic shifts of the carboxylate band frequency do not seem to follow any specific trend. The band shifts caused by Fe2+, Zn2+, Cu2+, Ag+, and Hg2+ do seem to trend downward with the period in which the element occurs. However, Cd2+ and Pb2+, although separately trending likewise, do not fall on the same trend-line as the former set of five ions. νsym(COO-) for several of the metal-carboxylate surfacecomplexes are sufficiently shifted in frequency from each other to allow the relative composition of two coadsorbed ionic species to be resolved. As example, we show that the individual contributions of surface-complexes of Pb2+ and Hg2+ in various proportions can be determined. The forgoing provided the underpinnings to the central experiment of this study: to visualize using SERS, the stepwise chromatographic separation of an initially equimolar solution of Pb2+ and Hg2+ (50 µM of each metal ion) along the length of a treated capillary. While the potential advantages of simultaneous detection along the entire length of a chromatographic column has been described previously,28,29 using a variety of on-column detection methods such as photoacoustic, fluorescence, and electrochemical detection, ours is the first study that actually monitors the chemical composition of the separation medium using a molecule specific spectroscopic technique (SERS) to identify the chemical species present. Referring to Figure 4a, a small sample of an equimolar Pb2+/Hg2+ solution (50 µM in each species) was injected near the inlet of a 100 mm long by 1 mm diameter capillary whose interior was prepared as described above. The sample formed a liquid column ∼2 mm long (corresponding to a volume of ∼1.6 µL). The sample was then moved to the initial point at “0 mm” in the capillary by injecting air into

FIGURE 3. (a) Frequencies of the band maxima of the ν(COO-) and δ(COO-) SERS bands. (b) Intensity ratios of those bands in the SERS spectra recorded when 1 mM solutions of the metal ions indicated were flowed through the capillary.

boxylate group. These are reported to occur at 1660, 1380, and 843 cm-1, respectively, corresponding to the stretching band of carbonyl (ν(CdO)), the symmetric stretching band of carboxylate (νsym(COO-)) and the deformation band of carboxylate (δ(COO-)).24-26 The first is associated with the carboxylic acid group and occurs only in acidic (pH ) 1.2) media. It is suppressed at pH values larger than the molecule’s pKa when the carboxylic group becomes deprotonated to form the carboxylate anion and in the presence of a strongly bonding cation that also drives the equilibrium toward carboxylate formation. In our spectra (in the absence of the metallic cations), we observe these bands to occur at 1696, 1389, and 843 cm-1. Notably, no band is observed in the ∼2580 cm-1 region corresponding to a vibration of the thiol (-SH) group, which is observed in the Raman spectrum of solid MBA. We interpret this as evidence for the fact that the thiol group sheds its hydrogen (to produce thiolate) when forming the adsorbate-Au bond. In the presence of metal cations, a number of notable changes (and lack of changes) are noted (Figure 2b-h). First, most SERS bands remain with very little changes when aqueous salt is injected into the capillary. The notable exceptions are the νsym(COO-) and δ(COO-) bands that shift in frequency and change in intensity and in band contour. Some of these changes are summarized in Figure 3. Additionally, the intensity of a band we observe at 409 cm-1 varies dramatically with cation. It is intense in the presence of Fe2+ and almost absent in the presence of Cd2+ and Hg2+. The assignment of this band is uncertain. Perhaps it is a torsional band with some contribution from carboxylate motion.24 In the presence of all cations but Cd2+, the value of νsym(COO-) is lower than it is in the presence of pure water. Also, the intensities of the νsym(COO-) bands are significantly © 2011 American Chemical Society

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FIGURE 5. (a) Points are the intensities of the Pb2+ (red) and Hg2+ (blue) surface carboxylate components (normalized to that of the band at 1584 cm-1) extracted by fitting a linear combination of the measured spectra of the individual ions coordinated to the surface carboxylates as described in the Supporting Information. Lines are calculated assuming competitive adsorption between the two ions for surface-bound carboxylate ligands. (The large scatter arises from the fact that the highly colored capillary introduces intensity variations from spot-to-spot along the capillary.) (b) The fit to the model is much better when the ratio of the SERS intensity components of the Pb2+ to the Hg2+ surface complex are compared to the calculated ratio. (The ratio eliminates common factors affecting the two intensity components equally.) The error bars reflect the spread between results recorded with two independent sets of capillaries fabricated and measured many weeks apart.

FIGURE 4. (a) Schematic of the capillary system used to separate Hg2+ and Pb2+ ions out of an equimolar solution of the two ions, detecting their relative concentrations along the length of the capillary at the positions indicated that are separated approximately by 10 mm. (b) SERS spectra recorded at the locations indicated. (c) Carboxylate stretching region. Intensities in both panels b and c were normalized to that of the 1584 cm-1 band that was used as an internal standard.

spectra to a linear combination of the νsym(COO-) bands recorded when the capillary was (independently) exposed to the individual ions; and the determined coefficients were taken to be measures of the component intensities. (This approach assumes that a given carboxylate interacts only with a single ion (as would be the case for bidentate chelation) and that there is little coupling between neighboring carboxylates so that the frequency of a carboxylate complexed with Pb2+ is not influenced by a vicinal carboxylate complexed to Hg2+.) The method used and the results obtained are described in greater detail in the Supporting Information and the results are plotted (points) in Figure 5a. The general trend is such that (except at the first three locations) the surface coverage of the Pb2+ increases, and that of the Hg2+ decreases as one advances from the point of entry of the capillary, inward, assuming that the SERS intensity is a monotonic measure of the surface coverage. The measured data show considerable scatter despite the high quality of the

the capillary from a syringe. A SERS spectrum was recorded. The sample was then moved to a point approximately 10 mm beyond the initial point, and a SERS spectrum recorded once again. The process was repeated until the νsym(COO-) SERS band corresponded to only one of the two metal ions as previously determined in the experiments described above. That occurred when the solution column had been moved to a point in the capillary ∼80 mm from the initial point. The nine resulting spectra are shown in Figure 4b, and an enlargement of the νsym(COO-) region of these spectra is shown in Figure 4c (along with the νsym(COO-) region of SERS spectra recorded with Pb2+ and Hg2+ separately). The SERS intensities of the Hg2+ and Pb2+ carboxylate species contributing to the observed band contour of the νsym(COO-) band were determined by fitting the measured © 2011 American Chemical Society

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SERS spectra. This is likely because the AuNPs-covered capillary is intensely colored so that even a small variation in colloid coverage could affect the intensity of the SERS intensity that passes through the colloid layer before it is collected by the spectrometer’s microscope objective. That this is likely the case is supported by the fact that the ratio of the Pb2+/Hg2+ components is a much smoother curve (as a function of position along the capillary) since common factors, such as a slightly variable point-to-point absorption, are eliminated in constructing the ratio (Figure 5b). At first glance, the increased adsorption of Pb2+ over that of Hg2+ seems counterintuitive since for metal acetates in solution the stability constants are log K(Hg) ) 10.1 and log K(Pb) ) 4.1.16 However, the following simple analysis indicates that what is observed is what is not unexpected. We assume that as the small volume is pushed along the interior of the carboxylated AuNPs lining the capillary, the metal ions will be removed from the solution phase and immobilized on the surface as a result of complexation with the carboxylate, reducing the concentration of the respective ion in proportion of what is adsorbed. For two species in solution, for example, a and b, competing for the same surface sites, and obeying a Langmuirtype adsorption equilibrium the surface coverages, respectively, na and nb (mol/unit area) are given by30

na ) Sβaca /(1 + βaca + βbcb)

(1a)

nb ) Sβbcb /(1 + βaca + βbcb)

(1b)

length equal to the length of the advancing sample column. For a capillary of circular cross-section of radius r, Acolumn/ Vcolumn ) 2/r. We will assume that the observed SERS intensities are proportional to na and nb. (This is an approximation since there are depolarization effects that modify the SERS intensities as a function of surface coverage.)31,32 The curves shown in Figure 5a,b were computed using eqs 1a, 1b and 2a, 2b and adjusting the values of βa, βb and the parameter 2S/r (taken to be a single adjustable parameter) and a scaling factor that includes the proportionality constant that connects surface coverage to SERS intensity. Equations 2a and 2b were solved iteratively by associating species a and b, respectively with Pb2+ and Hg2+, taking the initial concentrations of the two species to be equal, then computing new volume concentrations (ca,i+1 and cb,i+1) using eqs 2a, 2b. These computed concentrations were then assumed to be new values of ca,i and cb,i and new values of ca,i+1 and cb,i+1 were once again computed using eqs 2a, 2b for the next position along the capillary, and so on. The fit between observed and computed surface coverage is very good, especially when comparing the Pb2+/Hg2+ ratio. What is more, that fit assumes that the equilibrium constant for Hg2+ adsorption is 9-fold larger than that for Pb2+ adsorption. At first glance, it may seem counterintuitive that Hg2+ has greater affinity for the surface carboxylate, than does Pb2+, yet it is the Pb2+ complex that ultimately dominates the surface (e.g., at points 70 and 80 mm). But both simulation and observation concur that this is what would occur. Briefly, so much Hg2+ is extracted out of the solution at the early steps of the excursion of the liquid column through the capillary that ultimately the surface equilibrium is driven to favor the Pb2+ complex due to the very low concentration of Hg2+ remaining in the solution. (At early positions, e.g., 0, 10, and 20 mm, the surface concentration of Hg2+ does in fact exceed that of Pb2+. The Pb2+/Hg2+ equilibrium constant ratio is smaller than for metal acetates in solution. However, there is no reason to believe that the equilibrium constants for complexes adsorbed on the surface should be the same as those in solution.) In summary, we report the synthesis of a SERS-active separation medium, consisting of a dense layer of gold nanoparticles treated with 4-mercaptobenzoic acid, which are tethered to the interior of a capillary. We use the resulting column to follow the step-by-step chromatographic separation of the constituent ions in an originally equimolar solution of Pb2+ and Hg2+ until only one of them survives on the surface. The separation makes use of the differing equilibrium constants of lead and mercury carboxylato complexes, and the contribution of the individual lead and mercury species to the SERS spectrum relies on the unique SERS signatures of the two metal complexes in the COO- band spectroscopic region. The observed adsorption-desorption equilibrium along the capillary is shown to conform with the predictions of a

where S, βa, βb, ca and cb, are, respectively, the number of sites per unit area (expressed as mols per unit area), the Langmuir adsorption equilibrium constant for species a, and for species b, and the concentrations of a and b in solution. We assume that the ions contained in the small advancing liquid column achieve adsorption equilibrium before the SERS measurement is made. When the column is then advanced to the next location, it finds fresh surface but a reduced concentration of the two ions because some of the ions have been removed out of the solution by adsorption at the prior location. Hence, the concentration of ions a and b at location i + 1 along the capillary will be related to their respective concentrations at location i as follows

ca,i+1 ) ca,i - na,i(Acolumn /Vcolumn)

(2a)

cb,i+1 ) cb,i - nb,i(Acolumn /Vcolumn)

(2b)

in which Acolumn and Vcolumn are, respectively, the interior surface area, and the interior volume of the capillary for a © 2011 American Chemical Society

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model based on Langmuir adsorption-desorption equilibrium for the two surface species and their counterparts in solution. We show that it is the Pb2+ that survives the separation process as the sole surface species, despite the fact that Hg2+ complexes with COO- much more strongly than Pb2+. We show that this is because so much mercury is taken out of solution during the early separation steps that the surface equilibrium is ultimately driven toward adsorbed Pb2+. Acknowledgment. This work was supported by the Institute for Collaborative Biotechnologies through Grant DAAD19-03-D-0004 from the U.S. Army Research Office and made extensive use of the MRL Central Facilities at UCSB supported by the National Science Foundation under Award Nos. DMR-0080034 and DMR-0216466 for the HRTEM/ STEM microscopy. Supporting Information Available. Simulation and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2)

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