Phase Switching to Enable Highly Selective Activity-Based Assays

Oct 17, 2012 - detection whereby a soluble analyte is converted intentionally into a heterogeneous catalyst. This catalyst then reacts selectively wit...
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Phase Switching to Enable Highly Selective Activity-Based Assays Hemakesh Mohapatra and Scott T. Phillips* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: Traditional activity-based (or reaction-based) detection schemes rely on homogeneous reactions between an analyte and a substrate to provide a signal that is proportional to the concentration of the analyte. Selectivity in these assays is governed primarily by the ability of the desired analyte to react faster than other analytes with the substrate. In this Article, we describe a conceptually different approach toward activity-based detection whereby a soluble analyte is converted intentionally into a heterogeneous catalyst. This catalyst then reacts selectively with a substrate to provide the readout for the assay. This concept is particularly relevant to heavy metal detection, as demonstrated by a rapid and highly selective assay for palladium in which a soluble metal ion is converted in situ into a colloidal catalyst.

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attributes (such as high selectivity) that are available during a phase switching assay. This type of phase switching reaction provides excellent selectivity for the assay since the selectivity is governed both by the rate of formation of the heterogeneous catalyst and by the activity of the in situ-formed catalyst. Hence, this concept of rationally designing heterogeneous activity-based assays offers new opportunities to the analytical community in terms of (i) enhancing selectivity and sensitivity, (ii) decreasing assay times (certain heterogeneous reactions, particularly metal colloidcatalyzed reactions, are very fast),2 and (iii) providing new types of reactions that can be incorporated into the growing repertoire of activity-based detection schemes.3,4 The goal of this Article is to describe this general concept of phase switching in activity-based detection, to further show that a soluble analyte can be converted in situ into a heterogeneous catalyst, and to demonstrate the concept by detecting palladium quantitatively. Complete analytical details associated with the practical aspects of detecting palladium in real-world samples will be the subject of future studies. The assay depicted in Figure 1 is based on literature precedent that soluble palladium (either palladium(II) salts or ligand-stabilized zerovalent palladium) can be converted into palladium(0) nanoparticles when exposed to Et3SiH.5 These types of nanoparticles have been shown to catalyze the 1,4reduction of α,β-unsaturated esters and the hydrogenation of alkenes.5 Likewise, a variety of ligand-stabilized palladium nanoparticles has been used to reduce dyes in the presence of reducing reagents such as NaBH4, hydrazine, or ascorbic acid.6 On the basis of these examples, we reasoned that a convenient phase switching activity-based assay might be possible (i) if

his Article describes a phase switching assay in which soluble palladium(II) and ligand-stabilized palladium(0) both serve as the analyte. The assay detects palladium via intentional in situ autocatalytic conversion of soluble palladium in a sample into palladium colloids (Figure 1). In the same

Figure 1. Schematic of a phase switching assay for detecting palladium(0) and palladium(II). The assay requires only 3 min at 23 °C in 20:1 DMF/water (pH 6). During this time, the solution turns from yellow to lighter shades of yellow or even colorless depending on the concentration of the palladium analyte.

solution, these palladium colloids then catalyze the Et2SiH2mediated reduction of a commercially available pH indicator dye, thus providing a time-dependent turn-off colorimetric readout that is easy to see and quantify. The entire assay is fast (3 min), operates at room temperature and is open to the air, is specific for palladium (both palladium(0) and palladium(II)) over other metals (including platinum), uses only commercially available reagents, occurs under aqueous and organic conditions (the palladium could originate from either solvent), and is sufficiently sensitive to measure well below the governmentregulated threshold levels of palladium in drugs.1 More importantly, the model assay for palladium illustrates key © 2012 American Chemical Society

Received: September 6, 2012 Accepted: October 17, 2012 Published: October 17, 2012 8927

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Analytical Chemistry

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palladium(0) nanoparticles could be generated from soluble palladium in an aqueous sample; (ii) if these nanoparticles could be generated in situ; (iii) if the catalytic reduction of a commercially available pH indicator dye could be accomplished selectively by these nanoparticles rather than by other metal ions present in a sample; and most importantly, (iv) if the rate of reduction of the dye correlated with the initial concentration of palladium in a sample.

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RESULTS Initial Demonstration of a Phase Switching Assay. To test this idea of phase switching, we chose bromothymol blue as the indicator because it provides an unambiguous change from yellow to colorless when reduced at approximately neutral pH. Other triarylmethane dyes such as bromocresol green at pH 5.5 and phenol red at pH 6.0 also can be used. Our initial assay conditions involved adding 100 μL of an aqueous sample of palladium to a N,N-dimethylformamide (DMF) solution (2 mL) containing bromothymol blue (0.12 mM) and Et2SiH2 (5 μL). Since the assay employs pH indicator dyes, we buffered the aqueous sample to pH 6 using 0.2 M 2-(N-morpholino)ethanesulfonic acid prior to mixing it with the DMF solution to ensure that the color of the solution was independent of the pH of the original aqueous sample. In initial testing, we measured whether the assay conditions provided a time-dependent change in color of the solution when the aqueous sample contained 94.4 μM Na2PdCl4 (10 ppm palladium) (Note: 94.4 μM was the concentration prior to buffering and further diluting the aqueous sample in the assay solution). Figure 2a shows a distinct change in the UV/vis spectrum of bromothymol blue during the assay. In particular, the signal at 450 nm nearly disappeared over the course of 20 min, which corresponded to the solution changing from yellow to colorless. Moreover, the distinct isosbestic point at 378 nm suggests 1:1 conversion from bromothymol blue to a single colorless product. We isolated and characterized this product and found that the spectral data matched the expected product shown in Figure 1 (Figure S1, Supporting Information). Verification of this assignment was accomplished using a known reduction reaction for triarylmethane dyes,7 which involved treating bromothymol blue with zinc in acetic acid at 60 °C (compare Figures S1 and S2, Supporting Information). We next showed that the assay produces a color change that is dependent on the concentration of palladium in a sample: e.g., Figure 2b shows both dose- and time-dependent changes in color for four samples of palladium. Quantitative Phase Switching Assay. Changing the duration of the assay will increase or decrease the sensitivity of the test, but since the government-regulated threshold level of palladium in drugs is 10 ppm,1 we developed a calibration curve using a fast 3-min assay that provided a linear response at this concentration (Figure 3a). The calibration curve was obtained by measuring the absorbance of samples at 450 nm 3 min after combining the aqueous palladium samples with solutions of bromothymol blue and Et2SiH2. The limit of detection (LOD) for this 3-min assay is defined as the lowest value that we detected, which is 2 ppm. We then measured samples of known palladium concentration by comparing absorbance values for the test assays with the calibration curve; the results from these test samples (Figure 3b) indicate that the assay is sufficiently accurate to measure relevant concentrations of palladium in drugs.

Figure 2. Colorimetric response of the phase switching assay for palladium. (a) Time-dependent UV/vis absorbance spectra when an aqueous sample of 10 ppm Pd (i.e., Na2PdCl4) is exposed to the assay conditions (pH 6, 25 °C). In all figures, the reported concentration of palladium reflects the initial concentration of palladium in the aqueous sample before it was buffered and further diluted in the assay solution. (b) Photographs of the assays over time for different concentrations of Pd (i.e., Na2PdCl4).

Figure 3. A quantitative phase switching assay for palladium. (a) Calibration curve for a 3 min assay. The data points represent the averages of three measurements. (b) Assay results for test samples. The assays used the calibration curve in (a); the experimental values are the averages of three measurements.

Evidence for Conversion of Soluble Palladium into a Heterogeneous Catalyst. To demonstrate that the assay conditions switch the phase of palladium, we performed a series of physical organic experiments to first gain an understanding 8928

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X-ray spectroscopic (EDX) analysis of the colloids in the TEM image reveals the presence of both palladium and silicon (Figure 4d), consistent with the presence of silane-functionalized colloidal palladium catalysts. Unfortunately, we were unable to use dynamic light scattering to measure the distribution of particle sizes for the colloids since the high concentrations (100 ppm) of palladium required for the measurement (relative to the concentration of Pd used in the assay) led to quantitative precipitation of palladium aggregates. Selectivity for the Phase Switching Assay. To explore the selectivity of the phase switching assay, we measured the effects of various metal salts in comparison with palladium (Figure 5). In the absence of added metal (i.e., when

of the mechanism of the reaction. From these studies, we conclude the following: depending on the initial oxidation state of the palladium analyte, Et2SiH2 either reduces palladium(II) to silane-functionalized palladium(0) or converts ligandstabilized palladium(0) to silane-functionalized palladium(0) (i.e., an A → B process).8 This silane-functionalized palladium(0) facilitates autocatalytic surface growth of colloids (an A + B → 2B process)8 that are then capable of catalyzing the reduction of the triarylmethane dye. A number of experiments provided data to support this mechanistic hypothesis. First, the sigmoidal reaction kinetics shown in Figure 4a are indicative of an autocatalytic process;

Figure 5. Metal ion screen that demonstrates the selectivity for Pd(0) and Pd(II) in the 3 min assay. Assays corresponding to entries 2−5 and 7 used 0.188 mM of the metal salt while the assays corresponding to entries 6 and 8−24 used 1.88 mM of the metal salt. The assays were repeated three times; the bars show the average values, and the error bars represent the standard deviations from these averages.

Figure 4. Evidence for the mechanism of the assay. (a) Sigmoidal kinetics become apparent when the reaction slows as a consequence of the use of 1 ppm Pd (9.4 μM Na2PdCl4) as the analyte in the assay (black data). The gray data (top line) was obtained in the absence of Pd. (b) The addition of metallic mercury to an active assay (2 ppm Pd; 18.8 μM Pd(OAc)2) completely prevented further change in signal (gray data, top line), although vigorous stirring for 2 s after adding the mercury resulted in a brief increase in absorbance. When mercury was not added, but an identical assay procedure was followed, the reaction recommenced after a brief induction period (black data). (c) TEM image of palladium colloids formed during an active assay using 425 ppm Pd. The colloids were trapped on a TEM grid after a 2 min assay time. An expanded view of the TEM image is available in Figure S3, Supporting Information. (d) EDX analysis of the particles in (c) showing the presence of both silicon and palladium. The copper is from the TEM grid.

bromothymol blue is exposed only to Et2SiH2), no change in color was observed even after 12 h (Figure 5, entry 1). The assay is highly specific for palladium (both palladium(II) and palladium(0)) over many metals, including platinum and iron, both of which are common sources of false positives in other assays for palladium.11−22 Even when the assay is run using 1.88 mM Na2PtCl6 (367 ppm platinum; 10-fold more than the quantity of palladium used in Figure 5) for 6 h, no change in color was observed (entry 20, Figure 5). Gold, in contrast, does show some cross reactivity in the assay at high concentrations (1.88 mM) (Figure 5, entry 6), but at more realistic concentrations (i.e., 0.188 mM, or 37 ppm Au3+, entry 7), there is no response during the 3 min assay. Close inspection of Figure 5 reveals that not all palladium species provided a response in the 3 min assay. Specifically, the assay using Pd(PPh3)4 showed negligible change in color, even though other sources of palladium(0) (such as Pd2(dba)3) were effective. This difference in reactivity between the two sources of palladium(0) raised the concern that organic molecules in drugs and the environment might affect the performance of the model assay. While we have not performed an exhaustive screen for the effects of functional groups on the sensitivity of this first-generation assay, Figure 6a does reveal that 5000-fold excess quantities (by mass relative to palladium) of certain groups (such as pyridine) negatively affect the results of the 3min assay, while other functional groups (such as alcohols, phenols, and secondary amines) have a negligible impact. This data suggests that functional groups that associate tightly with soluble palladium (e.g., phosphines in Pd(PPh3)4) prevent the

the early lag phase of the reaction becomes evident when the rate of the assay is slowed using low concentrations of the palladium analyte (e.g., 1 ppm). In other systems, this kinetics profile was attributed to autocatalytic surface growth of metal nanoparticles/colloids.9 Second, to demonstrate that reduction of the triarylmethane dye is mediated by a heterogeneous catalyst, we carried out catalyst poisoning experiments using mercury(0) (Figure 4b), which is known to amalgamate heterogeneous metal catalysts but not homogeneous catalysts.8−10 Under these conditions, the reduction of the triarylmethane dye was prevented. Furthermore, transmission electron microscopy (TEM) of a sample taken directly from an example assay reveals ca. 3−5 nm diameter colloids (Figure 4c), which is consistent with the other evidence for a heterogeneous catalyst.9 Energy-dispersive 8929

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DISCUSSION

While our objective in developing a model assay for palladium was to illustrate the concept and benefits of phase switching, new assays to detect and measure palladium are useful in their own right. Palladium is used widely in a variety of industries including in the manufacture of consumer products (e.g., drugs, hydrogenated vegetable oils), automotive components (e.g., catalytic converters, spark plugs), electronics, medical devices (e.g., dental fillings), and jewelry. As a consequence, palladium contamination of both consumer products and the environment (as products degrade) is becoming an increasing concern, particularly since the metal may bind to and alter the function of proteins and nucleic acids.26,27 Traditional analytical methods for detecting palladium use expensive stationary instruments such as atomic absorption, Xray fluorescence, and plasma emission spectrometers.28 In many settings, however, the ideal detection scenario would involve low-cost, operationally simple, and mobile assays that can be performed quickly with high-throughput by untrained individuals. These types of assays may be useful for detecting palladium in (i) the environment, where palladium leaches from electronic waste and is lost by catalytic converters,1 and (ii) the pharmaceutical industry and other situations where quality control measures in manufacturing processes must be monitored.29 It is in these latter contexts that a fully developed phase switching assay for palladium might be most useful, particularly since it is rapid, uses commercially available reagents, and can be quantified by absorbance, which is a spectroscopic technique for which a number of inexpensive, commercial hand-held readers have been developed. It also should be noted that a number of recent reagents have been reported for detecting palladium to circumvent the need for expensive stationary instruments in these latter applications.11−22 These reagents fit into two categories: (i) reactionbased detection reagents, in which palladium is detected through a specific homogeneous chemical reaction; and (ii) supramolecular reagents, in which palladium binds to a detection reagent.3,4 In both approaches, some background signal arises from the presence of platinum (activity-based detection) or iron (supramolecular reagents). Two excellent recent examples include (i) the reaction-based fluorescent reagents prepared by Koide et al., which are capable of detecting palladium(II) selectively over palladium(0), and vice versa,12,13 and (ii) the supramolecular fluorescent and colorimetric reagent prepared by Peng et al. for detecting palladium(II).14 The proof-of-concept phase switching assay described in this paper has practical value since it exhibits exceptionally high selectivity for palladium over other metals, is extremely fast, and uses inexpensive, commercially available reagents. Moreover, the source of palladium can originate from organic or aqueous solution, and the sensitivity of the colorimetric assay is excellent (the limit of detection is 0.7 ppm). Ultimately, since the focus of this Article is on the concept of phase switching, more development work is needed before the proof-of-concept phase switching assay for palladium described herein reaches the level of sophistication of known assays.12,13,28

Figure 6. Effects of additives and a revised assay. (a) Effects of additives on the colorimetric response for the 3 min assay. (b) A revised assay that includes a 1 h incubation period of Et2SiH2 with the analyte, followed by a 3 min assay with bromothymol blue. The quantity of palladium was 10 ppm in all assays in (a) and 20 ppm in (b), each bar represents the average of three measurements per assay; the quantity of platinum in (b) was 184 ppm, and the additives in (a) were present at 5000 ppm.

palladium from forming a colloid during the 3 min assay, thus leading to false negative results. Revised Phase Switching Assay for Palladium. To address this issue, we modified the assay for situations in which false negatives might be expected. Specifically, we included a preincubation step in which the palladium sample was exposed to Et2SiH2 for 1 h prior to introducing bromothymol blue (the duration of the assay with bromothymol blue remained at 3 min). The preincubation step was included to provide sufficient time for any source of palladium (including those with tightbinding ligands) to convert into the colloidal palladium catalyst necessary for the assay. Using this protocol, even Pd(PPh3)4 can be detected in the assay (Figure 6b). This preincubation step does not appear to erode the selectivity: e.g., under the same assay conditions, a 5-fold higher concentration of either platinum(II) or platinum(IV) (relative to the quantity of Pd(PPh3)4 and Pd(OAc)2) shows no response (Figure 6b). Rationale for the Selectivity of the Phase Switching Assay. On the basis of the accumulated evidence provided by the kinetics, selectivity screens, and mechanistic data, we suggest that the selectivity of this phase switching assay is due, in part, to the catalytic activity of the palladium colloid. Other metals form colloids under the reaction conditions: e.g., TEM, EDX, and light-scattering experiments reveal that Na2PtCl6 forms silane-functionalized colloids of ca. 2−5 nm diameter (Figure S4, Supporting Information), but the selectivity experiments (Figure 5) indicate that the colloids are poor catalysts for the reduction reaction. Likewise, AuCl3 forms ca. 2 nm diameter colloids (Figure S5, Supporting Information), but these colloids also are poor catalysts (compared with the palladium colloids) for the reduction reaction, although they are more effective than the platinum colloids (compare entries 6 and 20 in Figure 5). The selectivity of the assay also depends on the rate of formation of the colloidal species: i.e., it is wellknown that surface growth of palladium colloids is exceptionally fast compared with other metals such as platinum.23−25

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CONCLUSIONS The results presented in this Article demonstrate that heterogeneous reactions can be used as an advantage for designing selective activity-based detection assays and should 8930

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be considered in the realm of the growing field of reactionbased analytical chemistry. The work also points to a new strategy of intentionally converting a soluble analyte into a heterogeneous catalyst to take advantage of the useful characteristics associated with a heterogeneous activity-based assay. These characteristics include access to fast assays, new modes of obtaining selectivity in an assay, and new types of reactivity that can be harnessed to detect an analyte selectively. Future studies will explore whether this phase switching concept can be employed in other contexts.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Arnold and Mabel Beckman Foundation, the Camille and Henry Dreyfus Foundation, Mr. Louis Martarano, and The Pennsylvania State University. S.T.P acknowledges support from the Alfred P. Sloan Research Fellows program. We thank Dr. Landy K. Blasdel for assistance in preparing the manuscript.

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EXPERIMENTAL SECTION Monitoring the Palladium Catalyzed Reduction of Bromothymol Blue by UV−Vis Spectroscopy (Figure 2). An aqueous solution of Na2PdCl4 (100 μL, 94.4 μM, [Pd] = 10 ppm) was diluted with aqueous 2-(N-morpholino)ethanesulfonic acid (MES) (100 μL, 0.2 M, pH 6.0), and an aliquot (100 μL) of the resulting solution was added to a quartz cuvette containing a mixture of bromothymol blue (2 mL, 0.12 mM in DMF) and Et2SiH2 (5 μL). No precautions were used to exclude air. The mixture was agitated using a vortex mixer for 2 s. The UV/vis absorbance spectra of the resulting mixture were obtained at regular intervals. Assay Condition for the Detection of Palladium (Figure 3). An aqueous solution of Na2PdCl4 (100 μL, 0− 150.4 μM, [Pd] = 0−16 ppm) was diluted with aqueous MES (100 μL, 0.2 M, pH 6.0), and an aliquot (100 μL) of the resulting mixture was added to a solution of bromothymol blue (2 mL, 0.12 mM in DMF) containing Et2SiH2 (5 μL). The combined solution was agitated for 2 s using a vortex mixer, after which the vial was left undisturbed for 2 min. The assay mixture was then transferred to a quartz cuvette using a glass pipet, and the absorbance at 450 nm was obtained 3 min after the addition of the palladium solution to the dye solution. Kinetics of the Palladium Catalyzed Reduction of Bromothymol Blue (Figure 4a). An aqueous solution of Na2PdCl4 (100 μL, 0 or 9.40 μM, [Pd] = 0 or 1 ppm) was diluted with aqueous MES (100 μL, 0.2 M, pH 6.0), and an aliquot (100 μL) of the resulting solution was added to a mixture of bromothymol blue (2 mL, 0.12 mM in DMF) containing Et2SiH2 (5 μL) in a quartz cuvette. The combined mixture was agitated for 2 s using a vortex mixer, and the absorbance at 450 nm was obtained at regular intervals. Screening of Metals to Determine the Selectivity for Palladium (Figure 5). A solution of metal salt (50 μL, 0.188 mM or 1.88 mM) was added to a mixture of bromothymol blue (2 mL, 0.12 mM in DMF), Et2SiH2 (5 μL), and MES buffer (50 μL, 0.2 M, pH 6.0). The reaction mixture was agitated for 2 s using a vortex mixer, after which the solution was left undisturbed for 2 min. The assay mixture was transferred to a quartz cuvette using a glass pipet, and the absorbance at 450 nm was obtained 3 min after addition of the metal salt solution to the dye solution.

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AUTHOR INFORMATION

REFERENCES

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S Supporting Information *

Additional experimental details, figures, and tabulated data. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. 8931

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