Portable Electrochemical Surface-Enhanced Raman Spectroscopy

Technical Note pubs.acs.org/ac

Portable Electrochemical Surface-Enhanced Raman Spectroscopy System for Routine Spectroelectrochemical Analysis A. M. Robinson, S. G. Harroun, J. Bergman, and C. L. Brosseau* Department of Chemistry, Saint Mary’s University, Halifax, Nova Scotia, B3H 3C3 Canada S Supporting Information *

ABSTRACT: A simple, portable electrochemical surfaceenhanced Raman spectroscopy (SERS) system is reported, consisting of a small benchtop Raman spectrometer, a laptop computer, and a portable USB potentiostat. Screen printed electrodes modified with silver colloidal nanoparticles are used as the SERS-active electrode, which exhibit long-term stability once prepared. Spectroelectrochemical analyses of paraaminothiophenol and melamine as model systems was conducted. In both cases, an increase in SERS signal is observed upon modulation of the applied voltage, indicating an inherent benefit of such a system wherein the surface charge can be easily tuned. Given the low cost, rapid analysis time, and good sensitivity of this system, this simple setup could be implemented for many on-site sensing applications, ranging from food and drug analysis to environmental monitoring and to chemical and biological warfare agent detection.

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reported by Li et al. whereby the authors used a portable Raman spectrometer and a laptop computer along with modified screen printed electrodes.31 Although the authors used both electrochemistry (to preconcentrate the analyte onto the silver surface) and SERS, they did not use both methods simultaneously, and thus, this system is not technically a portable spectroelectrochemical system. In this Technical Note, a simple, portable, and cost efficient electrochemical SERS system is reported. Once the analysis is complete, the electrode and cell are simply discarded after use, making this system ideal for on-site field applications where complex samples may adsorb irreversibly to the sensor surface and cleaning of glassware and/or electrodes may not be possible. In addition, this system is simple and affordable enough to be used in undergraduate teaching and research laboratories for basic spectroelectrochemical studies.

eal-time on-site monitoring of analytes is currently in high demand for fields such as environmental analysis,1−3 forensics,4−6 and homeland defense.7,8 Raman spectroscopy is ideally suited for this purpose given its easy portability, rapid analysis time, molecular fingerprinting capability, and nondestructive nature.9−12 To this end, several portable Raman spectrometers for field applications have been developed and are commercially available.13 Limitations restrict the usefulness of Raman spectroscopy for routine field analysis; however, such as weak signal intensity and interference from fluorescence. To circumvent these issues, surface-enhanced Raman spectroscopy (SERS) has typically been employed.14−17 Recent advances in the field of surface-enhanced Raman spectroscopy have resulted in a plethora of applications of this technique for field analysis, ranging from chemical warfare agent detection to food and drug additive analysis and to disease pathogen determination.18−21 As with SERS sensors, electrochemical sensors have also been developed and widely applied.22−25 Since the SERS-active surface is typically made of the coinage metals, such as copper, silver, or gold, it is natural to combine spectroscopy and electrochemistry in this case. Despite the clear advantages offered by a combined electrochemical SERS sensor, such as enhanced selectivity and signal intensity, examples of such sensors for field applications in the literature are limited.26 There are several examples in the literature of typical spectroelectrochemical studies for adsorbates on SERS active electrodes, including 2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole,27 benzenethiol,28 pyridine,28 rhodamine 6G, and other organic dyes,29 as well as DNA;30 however, none of these studies were done using simple, cost-effective, portable instrumentation and disposable electrodes/cells. The closest example of a portable quasi electrochemical SERS system was © 2011 American Chemical Society

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EXPERIMENTAL SECTION Spectroelectrochemical System. The Raman spectrometer used for these studies consisted of a DeltaNu (Intevac Photonics) benchtop dispersive Raman spectrometer equipped with an air-cooled CCD, 785 nm diode laser, and an optics extension tube. The power at the sample and aquisition times were typically 22.3−55.9 mW and 30−60 s, respectively. The spectrometer resolution was 4 cm−1. The potentiostat was a Pine Research Instrumentation portable USB Wavenow potentiostat. The student voltammetry cell, also available Received: July 7, 2011 Accepted: December 26, 2011 Published: December 27, 2011 1760

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Figure 1. Schematic setup of the portable electrochemical surface-enhanced Raman setup. The inset shows an SEM image collected for the silver nanoparticle working electrode surface.

Figure 2. (a) Electrochemical SERS spectra for a para-aminothiophenol self-assembled monolayer recorded in 0.1 M NaF as supporting electrolyte recorded from 0.0 to −1.0 V vs Ag/AgCl in 100 mV increments. Laser power at sample = 55.9 mW, acquisition time = 60 s. (b) Potential dependence of the 1427 cm−1 band for DMAB at 785 nm excitation. The intensity values are normalized with respect to the maximum intensity.

was a LEO1450VP SEM installed with an Oxford INCA200 EDS system. Reagents. para-Aminothiophenol, sodium citrate, and sodium fluoride were all purchased from Sigma Aldrich (St. Louis, MO) and used without further purification. Silver nitrate (99.9995%) was purchased from Alfa Aesar (Wardhill, MA). Melamine (99%) was purchased from Acros Organics (Morris Plains, NJ, USA). Skim milk was purchased from a local grocery store. All solutions were prepared using Millipore water (>18.2 MΩ cm).

from Pine Research Instrumentation, was used for the electrochemical cell. This simple cell design, which consists of a standard glass vial and a special adapter, is ideal for field analysis, since the vial itself is disposable. The SERS-active screen printed electrode was fashioned by depositing three 5 μL layers of Ag nanoparticles, produced as outlined below, onto the carbon working electrode surface of the screen printed electrodes (Pine Research Instrumentation). The Ag nanoparticle (AgNP) electrodes were allowed to dry completely before use and were found to give stable SERS signal for several months once dry. Preparation of Citrate-Reduced Colloids. Citrate reduced silver colloids were prepared using the standard Lee and Meisel preparation,32 having a peak absorption wavelength of ∼420 nm and a fwhm of ∼100 nm. After cooling, the colloidal nanoparticles were centrifuged 10 times (relative centrifugal force = 36 000g, 15 min per cycle) to concentrate the colloid. The inset to Figure 1 shows an SEM image of the AgNP electrode surface prepared as described above. The SEM

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RESULTS AND DISCUSSION Figure 1 shows a schematic diagram of the portable spectroelectrochemical system used in this work. This system consists of a small commercially available benchtop 785 nm Raman spectrometer (36 cm (l) × 25 cm (w) × 13 cm (h), 14 pounds) and a commercially available portable USB potentiostat (17.5 cm (l) × 9.5 cm (w) × 3.0 cm (h), 0.5 pound), both of which connect via USB to a laptop computer. The 1761

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Figure 3. (a) Electrochemical SERS spectra for spiked diluted skim milk sample, containing 2 mM melamine. Spectra were recorded from 0.0 to −1.2 V vs Ag/AgCl in 100 mV increments. Laser power at sample = 55.9 mW, acquisition time = 30 s. Asterisk denotes band due to citrate. (b) Electrochemical SERS spectra for diluted skim milk sample, in the presence (i) and absence (ii) of 0.4 mM melamine, recorded at −1.0 V vs Ag/ AgCl. Laser power at sample = 55.9 mW, acquisition time = 30 s.

electrochemical cell is a standard voltammetry cell supplied by Pine Research Instrumentation, and a screen-printed electrode (working electrode (WE) = carbon; reference electrode (RE) = Ag/AgCl; counter electrode (CE) = carbon) that has the working electrode surface modified with colloidal silver nanoparticles, as outlined above, is used as the SERS-active electrode. All voltages are reported versus the Ag/AgCl reference electrode. As an initial test of this portable electrochemical SERS system, the spectroelectrochemical behavior of two analytes was examined, p-aminothiophenol (pATP) and melamine, both of which were investigated over the wavenumber range from 200 to 2000 cm−1. pATP has already been extensively studied using electrochemical SERS, and a presentation of this data here serves only to illustrate how this simple portable system can provide reproducible data of equivalent quality to that previously reported in the literature for nonportable systems. For the melamine system, this work is the first example whereby portable electrochemical SERS was used for the in situ detection of melamine in milk, and a clear advantage of the ability to manipulate the surface charge is illustrated. para-Aminothiophenol is a commonly used SERS probe due to the fact that it can form a strong Ag−S bond with the substrate surface and it exhibits an intense SERS signal. In addition, pATP is gaining in significance due to its application in molecular electronics.33,34 Recent studies have shown that the SERS spectrum of pATP is actually a signal composed of two molecules on the surface, pATP and p,p′-dimercaptoazobenzene (DMAB), the latter of which forms as a result of a catalytic coupling reaction between adjacent pATP molecules on the silver nanoparticle surface.35,36 After a 60 min incubation in a 20 mM pATP ethanolic solution, the SERS-active SPE was rinsed with ethanol and placed in the spectroelectrochemical cell containing 0.1 M NaF as the supporting electrolyte. Figure 2a shows the electrochemical SERS spectra recorded for pATP, stepping in 100 mV increments in the cathodic direction from 0.0 to −1.0 V. The electrochemical SERS spectra presented

here are consistent with electrochemical SERS spectra obtained by Osawa et al. at different excitation wavelengths,37 with the current spectra being of equal, if not better, quality, achieved with the portable electrochemical SERS system as described above. In fact, Figure 2b shows the relative intensity of the 1427 cm−1 band for the DMAB species as a function of applied voltage, which is consistent with the wavelength-dependence trend observed by Osawa for this band, and is further evidence of the charge-transfer (CT) mechanism functioning for this vibrational mode.37 This result demonstrates that this novel portable system can provide excellent electrochemical SERS spectra, consistent with what can be found in published literature, despite the fact that it uses simple, disposable screen printed electrodes and a nonthin layer spectroelectrochemical cell. To test a system of real field interest, melamine contamination was analyzed in spiked milk samples. Since it is 67% nitrogen, melamine was added and, in some cases, continues to be added, to foodstuffs to boost apparent nitrogen content, thus causing that food item to appear particularly protein-rich. Melamine and its derivative cyanuric acid are known to form complexes of low solubility, and as such, intake of melamine has been linked to kidney stones and other problems.38−40 Some current methods for analyzing melamine contamination include HPLC,38 LC-MS,39 MALDI-MS,40 and ELISA,41 to name a few; however, these methods are often costly and/or time-consuming. Recently, SERS has been used to analyze for melamine in various foodstuffs, including liquid milk and protein pharmaceutical preparations.42,43 The normal Raman spectrum for a pure sample of melamine powder is shown in Figure S-1 in the Supporting Information. The most prominent band in the Raman spectrum is at 676 cm−1, with the second most intense band arising at 985 cm−1; both of these bands are due to ring-breathing modes of the triazine ring.44 Figure S-2 (Supporting Information) shows the electrochemical SERS spectra for a 10 mM melamine solution in 0.1 M NaF. Here, the band that was observed at 676 cm−1 in 1762

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the normal Raman spectrum is shifted to higher wavenumber (705 cm−1) once present at the Ag/solution interface, as is consistent with literature findings.45 Since the band at 705 cm−1 (which shifts to 680 cm−1 as the voltage becomes more negative) was the most intense feature in the SERS spectrum, it was chosen as the marker band for analyzing milk samples spiked with melamine. A sample of skim milk was spiked with a weighed amount of solid melamine, dissolved in a small amount of ethanol, to give a total melamine concentration of 10 mM. This milk sample was then centrifuged at 10 000g for 30 min, after which the supernatant was removed and placed into a clean centrifuge tube and then centrifuged for a further 15 min at 5000g. Next, a 0.50 mL aliquot of this supernatant was added to 2.0 mL of 0.1 M NaF to give a 2 mM total concentration of melamine, and electrochemical SERS spectra were recorded. The same analysis was done for a sample of milk that was not spiked with melamine, for comparison. Figure 3a shows the electrochemical SERS spectra recorded for the spiked, diluted milk sample. Here, at open circuit potential (ocp), the citrate signal is the main observable feature, with the 705 cm−1 band for melamine also being apparent. As the voltage is gradually made more negative, the melamine band at 705 cm−1 grows in intensity and appears most intense at −0.4 V, and then at more negative voltages, a number of intense bands appear in the range from 1200 to 1700 cm−1, due mainly to the protein and lipid components of the milk.46,47 At −1.0 V, the main melamine band reappears, now shifted to 680 cm−1, along with the strong bands from the lipid and protein components. A comparison of the electrochemical SERS in the presence and absence of 0.4 mM melamine (Figure 3b) shows that, at −1.0 V, the 680 cm−1 band for melamine can easily be detected, with no interference from residual milk components. SERS signal for further dilutions recorded at −0.4 V (data not shown), where the melamine signal was greatest, allowed for detection of the melamine signal down to 5 ppm. This experiment demonstrates the usefulness of combining electrochemistry and SERS for in situ determination of the presence of melamine in a complex matrix such as milk.

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Technical Note

ASSOCIATED CONTENT

* Supporting Information S

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone (902) 496-8175. Fax (902) 496-8104.

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ACKNOWLEDGMENTS The authors are grateful to the Natural Sciences and Engineering Research Council of Canada and Saint Mary’s University Faculty of Graduate Studies and Research for funding. A.M.R. acknowledges a SMU science research award for funding. Dr. Xiang Yang from Saint Mary’s University is acknowledged for help with operation of the SEM.

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CONCLUSIONS

This Technical Note reports a novel and affordable portable electrochemical SERS system, wherein electrochemical and SERS data can be recorded simultaneously in situ, for on-site analysis. Consisting of a lightweight benchtop Raman spectrometer, a laptop computer, and a portable USB potentiostat, this system is compact and easily portable from lab to lab or from lab to field. The use of modified screen printed electrodes and simple glass vials makes this system extremely cost-effective, convenient, and user-friendly, allowing analysis to be done on complex samples without the worry of permanent fouling of the electrochemical cell and its components after measurement. Demonstrated herein are spectroelectrochemical studies of two model systems, paraaminothiophenol, a common SERS probe, and melamine, an industrial chemical recently reported in some imported foods with demonstrated toxicity. The pATP system shows very good electrochemical SERS signal that is consistent with published data, and the added tunability afforded by application of a potential is demonstrated, particularly for the rapid detection of melamine in milk. 1763

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