Letter pubs.acs.org/ac
Unraveling the Mysterious Role of Palladium in Feigl bis(dimethylglyoximate)nickel(II) Spot Tests by Means of Confocal Raman Microscopy Manuel F. G. Huila,† Nikolas Lukin,† Andre L. A. Parussulo,† Pedro V. Oliveira,† Pedro K. Kyohara,‡ Koiti Araki,† and Henrique E. Toma*,† †
Instituto de Química, Universidade de São Paulo, S. Paulo, SP, Brazil Instituto de Física, Universidade de São Paulo, S. Paulo, SP, Brazil
‡
ABSTRACT: The acid protection effect promoted by traces of PdCl2 in [Ni(dmgH)2] spot tests was elucidated from confocal Raman microscopy imaging, which revealed the formation of protecting layers of [Pd(dmgH)2] closing the extremities of the [Ni(dmgH)2] filaments.
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ince its introduction by Tschugaeff1,2 in 1905, dimethylglyoxime has become universally accepted as the most important and most widely used analytical reagent for the determination of nickel and palladium.3−7 In 1937, Fritz Feigl published the first English version of his book “Qualitative Analysis by Spot Tests” showing the unlimited possibilities of exploring the selective and sensitive reactions of metals and ligands at the microscale.8,9 Such tests, carried out on a filter paper, became very popular because of their practicality in chemical analysis, quality control, and forensic chemistry.10−12 One of the Feigl most remarkable spot tests is the analytical detection of Pd2+ using the red bis-dimethylglyoximatenickel(II) complex, [Ni(dmgH)2].13 This complex is not stable in the presence of acids, and its red color rapidly vanishes in an acidic environment. However, Feigl observed that the addition of traces of Pd2+on a filter paper, containing the [Ni(dmgH)2] complex, was capable of preserving the color of the red stain against diluted acids. Such effect was successfully explored for analytical purposes, allowing the identification of 0.05 μg Pd2+ at 1 ppm level.9,13 According to Feigl, a layer of acid-resistant [Pd(dmgH)2] should be formed, protecting the underlying [Ni(dmgH)2] against dissolution in the acid.13 However, how could such tiny amounts of Pd2+ exert an effective protection on [Ni(dmgH)2]? The first clue for understanding this point came from the observation of regular wires in the optical and electronic microscopy images of the [Ni(dmgH)2] complex precipitated on glass or FTO substrates (Figure 1A). Such characteristics were also observed on filter paper and were preserved after the treatment with 10−2 mol L−1 HCl in the presence of traces of Pd2+ ions (Figure 1B). According to the X-ray diffraction studies published by Godycki and Rundle14 in 1953, [Ni(dmgH)2] crystallizes from organic media as orthorhombic crystals, where the complex adopts a planar geometry with the two dmgH ligands in a macrocyclic configuration kept by intramolecular hydrogen bonding. (Figure 2). © 2012 American Chemical Society
In the crystal, the macrocyclic complex is arranged in a stacked configuration, kept by hydrophobic interactions and a weak metal−metal bond.14,15 This structure can be simulated theoretically, as illustrated in Figure 3, using the MM+/ ZINDO-1 methods, confirming the existence of a metal−metal bond, and the stacking of the successive macrocyclic rings, alternated by 90° rotation. The [Ni(dmgH)2] and [Pd(dmgH)2] complexes absorb at 548 and 460 nm, respectively (Figure 4). Nakamoto et al.16 attributed the visible band of [M(dmgH)2] as a mixture of metal−metal and metal-to-ligand charge transfer transitions, on the basis of the analysis of the resonance Raman profiles. This interpretation was in agreement with previous MO calculations showing that the origin of the visible band is not restricted to M-M dz2 orbitals but also involves the participation of the αdiimine ligand skeleton.17,18 The Raman spectra of [Ni(dmgH)2] exhibits characteristic strong peaks at 1352 and 1510 cm−1, ascribed to contributions of normal modes ν(C−CH3) + ν(NO) and ν(CN), respectively.16,19 These peaks are strongly enhanced at 532 nm, because of the resonance Raman effect. The same peaks can be observed in the Raman spectra of [Pd(dmgH)2] at 1333 and 1475 cm−1, which are strongly enhanced at the 488 nm excitation wavelength. Therefore, the differentiation of the [Ni(dmgH)2] and [Pd(dmgH)2] species can be conveniently carried out on the basis of their resonance Raman spectra. In order to investigate the resonance Raman spectra of the microwires of [Ni(dmgH)2] and [Pd(dmgH)2], the samples were prepared on glass slips, and washed with ethanol in order to remove all the reagents in excess. The Raman spectra were recorded as shown in Figure 5. Pursuing the understanding of the Pd2+ ion protection mechanism, the [Ni(dmgH)2] fibers were generated on a glass Received: February 2, 2012 Accepted: March 8, 2012 Published: March 8, 2012 3067
dx.doi.org/10.1021/ac3003352 | Anal. Chem. 2012, 84, 3067−3069
Analytical Chemistry
Letter
Figure 4. Reflectance spectra of [Ni(dmgH)2] and [Pd(dmgH)2] on Whatman filter paper.
Figure 5. Resonance Raman spectra of [Ni(dmgH)2] and [Pd(dmgH)2] microfibers (note the contrasting magnification factors) collected with a 100× (N.A. = 0.8) objective focused on a single fiber.
Figure 1. SEM (A) of [Ni(dmgH)2] wires on FTO (fluorine dopped tin oxide) and (B) of the wires on the fibers of a paper substrate after the treatment with traces of Pd2+ ions and HCl 0.01 mol L−1. This sample was settled on an ITO (indium tin oxide) conductor glass using a carbon ribbon and coated with a 35 nm thickness layer of gold.
dmgH2 (10 mmol L−1). After a few minutes, the sample was carefully washed with ethanol in order to remove the excess of PdCl2, dmgH2, and free [Pd(dmgH)2] and examined using the confocal Raman microscope. Typical results are shown in Figure 6. In the optical image (Figure 6A), one can only see the fibers, with no distinction between the [Ni(dmgH)2] and [Pd(dmgH)2] species. A very similar image has also been obtained by scanning electron microscopy (SEM). However, using the laser excitation wavelength of 488 nm, the Raman signals of the minority [Pd(dmgH)2] complex were strongly enhanced (by a factor of 6), with respect to those of the majority [Ni(dmgH)2] complex. This allows one to map the distribution of the Pd(dmgH)2] complex from the enhanced intensity of the 1333 cm−1 peak, concomitantly with the [Ni(dmgH)2] species, monitored at 1352 cm−1. In this way, it is possible to access the precise distribution of these two species in the fibers, as shown by the combination profiles (for the 1352 and 1333 cm−1 peaks), expressed in red and green colors respectively, in Figure 6B. Therefore, according to the confocal Raman microscopy images, the [Pd(dmgH)2] wires are formed at the [Ni(dmgH)2] wire ends and also apparently, at some lateral spots. Actually, as shown in the SEM image (Figure 1A), the microscopic fibers are bundle of nanofibers, some of them of smaller size. Their exposition is responsible for the formation of the lateral spots. This is better illustrated in Figure 6C. In other words, the unprotected extremities of the [Ni(dmgH)2] wires are the main target for
Figure 2. Molecular structure of [Ni(dmgH)2].
Figure 3. Linear stacked structure of [Ni(dmgH)2]: Side view (left) and top view (right); see color codes in Figure 2.
plate, washed with ethanol, and treated with a drop of 10−5 mol L−1 solution of PdCl2 and a drop of an ethanolic solution of 3068
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Analytical Chemistry
Letter
(electron multiplier CCD technology). Solid state Nd:YAG (λ = 532 nm) and Ar ion (λ = 488 nm) lasers were employed in the resonance Raman measurements. The spectral images were obtained by scanning the sample in the x,y direction using a piezo driven xyz feed-back controlled scan stage, collecting one spectrum per pixel and using a Nikon objective (100× NA= 0.8). The Raman images were generated using the integrated peak intensities rather than the peak heights, in order to ensure a best contrast and resolution.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Brazilian agencies FAPESP and CNPq and from PETROBRAS. REFERENCES
(1) Tschugaeff, L. Ber. Dtsch. Chem. Gesel. 1905, 38, 2520. (2) Tschugaeff, L. Z. Anorg. Chem. 1905, 46, 144. (3) Brunck, O. Z. Angew. Chem. 1907, 20, 824. (4) Hemmingsen, J.; Larkin, D.; Martin, T. Anal. Chem. 1986, 58, 2088. (5) Gordon, L.; Ellefsen, P. R.; Wood, G.; Hileman, O. E. Jr. Talanta 1966, 13, 55. (6) Tubino, M.; Queiroz, C. A. R. Anal. Chim. Acta 2007, 600, 199. (7) Matias, F. A. A.; Vila, M. M. D. C.; Tubino, M. Sens. Actuators, B: Chem. 2003, 88, 60. (8) Feigl, F. Qualitative analysis by spot tests; Elsevier: Amsterdam, 1938. (9) Feigl, F.; Anger, V. Spot Tests in Inorganic Analysis, 6th ed.; Elsevier: Amsterdam, 1972. (10) Svehla, G. Vogel’s Qualitative Inorganic Analysis, 7th. ed.; Prentice-Hall: Upper Saddle River, New Jersey, 2010. (11) Jungreis, E. Spot Test Analysis; Wiley Interscience: New York, 1985. (12) Thyssen, J. P.; Skare, L.; Lundgren, L.; Menn, T.; Johansen, J. D.; Maibach, H. I.; Liden, C. Contact Dermatitis 2010, 62, 279. (13) Feigl, F. J. Chem. Educ. 1943, 20, 298. (14) Godycki, E.; Rundle, R. E. Acta Crys. 1953, 6, 487. (15) Calleri, M.; Ferraris, G. Acta Crys. 1967, 22, 468. (16) Nishida, Y.; Kozuka, M.; Nakamoto, K. Inorg. Chim. Acta 1979, 34, L273. (17) Ohashi, Y.; Hanazaki, I.; Nagakura, S. Inorg. Chem. 1970, 11, 2551. (18) Anex, B. G.; Krist, F. K. J. Am. Chem. Soc. 1967, 89, 6114. (19) Szabo, A.; Kovac, A. J. Mol. Struct. 2003, 651, 547. (20) Kotrly, M.; Turkova, I. Advanced Microscopy Technologies for defense, Homeland Security, Forensic, Life, Environmental and Industrial Sciences. Book Series: Proceedings of SPIE, 2011, Orlando, FL, Vol. 8036, p 804608, DOI 10.1117/12.887850.
Figure 6. Optical image (A) and magnification of the selected rectangle with Raman mapping of the 1352 cm−1 [Ni(dmgH)2] peak and the 1333 cm−1 [Pd(dmgH)2] peak, presented as a combination profile (B) where the 1352 and 1333 cm−1 signals are simultaneously shown in red and green colors, respectively. The laser excitation wavelength of 488 nm allowed one to enhance the [Pd(dmgH)2] Raman signal, improving the image contrast. (C) Pictorial illustration of the bundle of fibers.
the attack of acids or Pd2+ ions. The external sides are protected by the methyl groups (Figure 2) exhibiting an hydrophobic character which facilitates the association of the fibers in bundles, increasing their chemical and physical resistance. This finding explains how a minor amount of Pd2+ can really protect the [Ni(dmgH)2] complex against acids, solving a mystery which persisted for more than 5 decades in the literature. In addition, it provides an interesting example of application of modern methods of microscopy in Analytical Chemistry.20
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EXPERIMENTAL SECTION UV−vis spectra were recorded on a FieldSpec FS3 fiber optics spectrophotometer from Analytical Spectral Devices (350 to 2500 nm). SEM images were recorded on a Jeol JSM 840A or a JSM-7401F field emission scanning electron microscopes. Confocal Raman measurements were performed with a WITEC alpha 300R microscope equipped with an EMCCD detector 3069
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