New Approach for the Surface Enhanced Resonance Raman

Article pubs.acs.org/ac

New Approach for the Surface Enhanced Resonance Raman Scattering (SERRS) Detection of Dopamine at Picomolar (pM) Levels in the Presence of Ascorbic Acid Murat Kaya† and Mürvet Volkan*,‡ †

Department of Chemical Engineering and Applied Chemistry, Atilim University, TR-06800 Ankara, Turkey Department of Chemistry, Middle East Technical University, TR-06531 Ankara, Turkey

‡

S Supporting Information *

ABSTRACT: The development of a novel surface-enhanced resonance Raman scattering (SERRS) platform that allows fast and sensitive detection of dopamine (DA) has been reported. The iron-nitrilotriacetic acid attached silver nanoparticle (Ag− Fe(NTA)) substrate provides remarkable sensitivity and reliable repeatability. The advantages of both the surface functionalization for specific analytes and the SERRS are integrated into a single functional unit. While the silver core gives the necessary enhancing properties, the Fe-NTA receptors can trap DA adjacent the silver core and the NTAFe-DA complex formed provides resonance enhancement with a 632.8 nm laser. DA could be detected in pM level without any pretreatment with a reliable discrimination against AA, by utilizing low laser power (10 mW) and short data acquisition time (10 s). The high sensitivity along with the improved selectivity of this sensing approach is a significant step toward molecular diagnosis of Parkinson’s disease.

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systems of interest while performing ultrasensitive chemical and biological detection.9−12 The engineering of substrates to generate high-quality SERS and SERRS signals from specific analytes is still a hot topic.13−20 By decorating the surface of the SERS substrate with molecular systems that are capable of trapping the analyte of interest electrostatically,21−23 chemically,24−27 or mechanically28 and keeping it proximal to the metal surface,29,30 one can target the detection of a single species present in a complex sample mixture at nano- to femtomolar level. It is well-known that Fe(III)-catechol complexes have exceptional stability and exhibit broad ligand-to-metal charge transfer complex in the visible region. Fe(catechol)33− and Fe(catechol)2 complexes have charge transfer bands at 490 and 570 nm, respectively, and a 532 nm laser31 has been used for the resonance Raman enhancement of Fe(catechol)33−. DA is a catecholamine and the resonance enhanced SERS spectrum of Fe(III)tris(dopamine), Fe(DA)−3, acquired with a green laser, has also been reported. In charge transfer complexes, the ligand-to-metal charge transfer band energies depend on the nature of the ligands. The electron-donating and electronwithdrawing properties of the substituents on the catecholate modulate the Lewis acidity of the ferric center and cause spectral shifts in the absorption spectra of catecholate

he demand for ultrasensitive detection methods for biological and chemical sample screening is an important issue in disease diagnosis. In recent years, there has been considerable interest in developing methods for detecting the neurotransmitting molecules from neurons deep inside the brain. Dopamine (DA) is one of the most significant neurotransmitters because of its role in the functioning of the cardiovascular, renal, hormonal, and central nervous system.1,2 The measurement of DA in biological systems is therefore important but complicated by the low basal concentrations (0.01−1 μM) that are encountered in the extracellular fluid of the central nervous system and the presence of several interfering compounds. Among those, ascorbic acid is of particular importance especially for electrochemical detection of dopamine. Ascorbic acid and dopamine are oxidized at nearly the same potential, which results in overlapping voltammetric responses irrespective of the electrode materials.3 Furthermore ascorbic acid concentration (0.1 mM) is normally tree orders of magnitude higher than the concentration of dopamine.3 Vibrational spectroscopy, on the other hand, easily distinguishes among substituted benzenes, such as catecholamines and their metabolites. Structurally unrelated species, such as ascorbate, will have different vibrational spectra altogether. Therefore, interference problems are expected to be greatly reduced or eliminated in Raman spectroscopic measurements.4 Surface-enhanced Raman and resonance Raman scattering methodologies (SERS and SERRS) are sensitive spectroscopic tools5−7 appropriate for attaining structural information8 on the © 2012 American Chemical Society

Received: April 19, 2012 Accepted: August 10, 2012 Published: August 10, 2012 7729

dx.doi.org/10.1021/ac3010428 | Anal. Chem. 2012, 84, 7729−7735

Analytical Chemistry

Article

Scheme 1. Schematic Representation for the Production of Ag−Fe(NTA) Nanoparticles and Possible Cross-Linking between Silver Nanoparticle and Fe(NTA) Complex

Figure 1. Schematic representation for the use of Ag−Fe(NTA) substrate as molecular traps for surface-enhanced resonance Raman scattering (SERRS)measurement of dopamine: (a) Fe(NTA) bound AgNP, (b) DA molecule, (c) NTA-Fe-DA complex on AgNP, (d) SERS spectrum of Fe(NTA) bound AgNP substrate, and (e) SERRS spectrum of 1 × 10−5 M DA solution on Fe(NTA) bound AgNP substrate.

complexes (Cox). Nitrilotriacetic acid (NTA) is one of the most widely used and studied organic chelating agents. Carboxylate donor groups containing a central nitrogen atom enable tetradentate chelation and, consequently, hexacoordinated metals such as iron can form 1:1 and 1:2 complexes with NTA. Fe(NTA)(catechol) 2− has a band at 610 nm. Replacement of two bidentate catechol ligands in Fe(catechol)33− by three carboxylate ligands and an amine group increases the Lewis acidity of Fe, redshifts the spectrum, and permits resonance enhancement of Fe(NTA)(catechol)2− with red lasers such as the He−Ne laser.32,33 Herein we report a new approach for the picomolar detection of DA in the presence of ascorbic acid (AA) utilizing surface enhanced resonance Raman scattering (SERRS) by

using a NTA-iron-modified silver nanoparticle (Ag−Fe(NTA)) SERRS probe. Hence, the advantages of both the surface functionalization for specific analytes and the SERRS are integrated into a single functional unit. While the silver core gives the necessary enhancing properties, the NTA-Fe receptors can trap DA adjoining the silver core and the NTA-Fe-DA complex formed provides resonance enhancement with a 632.8 nm laser. The usage of a long wavelength, He−Ne laser offers a good compromise between high SERRS intensity and less sample damage. The analytical performance of the method with respect to sensitivity and selectivity is presented and discussed in detail. 7730



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RESULTS AND DISCUSSION Preparation and Characterization of Ag−Fe(NTA) Substrate. Colloidal silver was prepared by aqueous reduction of silver nitrate with trisodium citrate using the method proposed by Lee and Meisel.34 The plasmon absorption maximum of the prepared silver colloid was located at 410 nm. The iron(III) nitrilotriacetate moiety “Fe(NTA)” is readily obtainable in aqueous solution by dissolving Fe(NO3)3 and NTA (Nα,Nα-Bis(carboxymethyl)-L-lysine hydrate) in water to achieve a molar ratio of 1:1 of Fe(III) to NTA (10−2 M). The pH of the Fe(NTA) solution was adjusted to 7.0 with 1 M NaOH prior to the treatment with AgNPs. This pretreatment was essential for achieving the complex formation. The complex was then immersed in an aqueous dispersion of AgNPs. After the addition of Fe(NTA) to freshly prepared citrate-stabilized AgNPs, the nanosurfaces were modified by Fe(NTA) through the NH 2 arm of the NTA (Scheme 1), (Supporting Information, Experimental Section). The structural characterization of surface-modified AgNPs was performed with Field Emission Scanning Electron Microscope (FE-SEM), (Supporting Information, Figure S1a). The number-length (arithmetic) mean size of silver nanoparticles (diameter) was measured as 31 ± 5 nm by sampling 100 nanoparticles randomly. The qualitative elemental composition of the prepared Fe(NTA)-modified AgNPs was identified with energy-dispersive X-ray analyses (EDX) (Supporting Information, Figure S1b). Fe(NTA) complex incorporation onto the silver nanoparticles was also monitored by infrared (IR) and inductively coupled plasma optical emission (ICP-OES) spectroscopy. (Supporting Information, Figure S2). SERRS Studies. Taking advantage of the molecular trapping properties of the prepared substrate, a simple experiment was designed, which is shown in Figure 1, where He−Ne (632.8 nm) laser was used as a light source. As shown in Figure 1, the SERRS intensity at 1480 cm−1 was assigned to the ring stretching vibration (υ19b) contributed mainly from the stretching of the carbon−carbon bond to which the oxygens are attached. The band at around 1270 cm−1 was assigned to the stretching of the catechol carbon−oxygen stretching, C−O bond (υC−O). The remaining bands are attributable to various other ring stretching vibrations.4,31−33 Thus the bands observed in the dopamine spectrum, at 1150, 1326, 1425, and 1567 cm−l can be identified with benzene modes υ15, υ3, υ14, υ19a, and υ8a, respectively.35 Raman bands of dopamine, iron−dopamine and iron-NTA-dopamine complexes in the 1100−1600 cm−1 range are listed in Table 1. Fe-NTA modification improved the selectivity and affinity of the silver nanoparticle surface toward dopamine and the stability of the captured dopamine on the surface. Another significant aspect was the strength of bands observed in the

corresponding spectra, particularly for the stretching vibration at 1480 cm−1. This fact was investigated by comparing the spectrum of 1 × 10−5 M DA acquired with Ag−Fe(NTA) substrate with those of 0.5 M DA and iron-DA complex [Fe(DA)2] obtained with silver colloid as SERS substrate. These three detection strategies are shown schematically in Figure 2 together with UV−vis spectra of aqueous solutions of DA, Fe(DA)2, and Fe(NTA)DA complexes. For the complex formation, Fe(III) and DA solutions were mixed in 1:2 ratios under neutral conditions. The He−Ne laser was not powerful enough to obtain the SERS spectrum of 0.5 M DA solution utilizing bare silver nanoparticles as the substrate (Figure 3A). Greater spectral enhancement was achieved by complexation of DA with ferric ions under neutral conditions due to the broad charge transfer absorption band centered on 570 nm convenient for resonance enhancement with a 632.8 nm laser. A good quality spectrum corresponding to the formed iron(III)bis (DA) complex was acquired with silver nanoparticles (Figure 3B). This spectrum and the DA spectrum obtained with Ag−Fe(NTA) substrate (Figure 3C) have similar spectral characteristics. The vibrations in both spectra were assignable to the same catecholate ring modes or C−O stretches of DA. The enhancement of the same ring vibration (υ19b) at 1480 cm−1 more than the others in these two spectra led to the conclusion that strong interactions were occurring between the nanoparticle surfaces and the catechol hydroxides on the molecule to promote the same specific orientation of the DA. However, the intensity increased approximately a hundred-fold when Ag−Fe(NTA) substrate was used. At this point several different contributions were taken into account. The first was the achievement of more favorable superposition of the electronic transition of the Fe(NTA)DA (∼λmax = 610 nm) (Figure 2c) with the energy of the incoming laser (632.8 cm−1) or the scattered light compared to that of iron DA complex ((∼λmax = 570 nm) (Figure 2b). A red shift of around 40 nm in the absorption band of the iron center results in a better resonance surface enhanced spectrum reflecting the intense vibrational modes associated with the DA group. The second was correlated with the preferred orientation of the adsorbate to the surfaces of the substrates. In SERS, enhancement of a given mode is the best when it is close and normal to the surface of the substrate.36 In iron-NTA-catechol complexes, NTA and catechol act as tetradentate and bidentate ligands respectively to yield a distorted octahedral geometry having catechol hydroxides at the axial and equatorial positions.37 In the case of Ag−Fe(NTA) substrate, a NTA group was appended to the silver surface, hence keeping the DA group in close proximity and orthogonal to the surface. The third was the bonding strength between the adsorbate and the substrate surface, which affects the number of molecules bound or in contact with the substrate and the time spent in that plasmon environment.38 The iron(III) bis(DA) and iron(III)NTA(DA) complexes have similar stability constants of logβ2 =35.5 and logβ=32.5 respectively. Although each iron complex brings two DAs to the surface, the negative charge of Fe(DA)2−1 complex might adversely affect the adsorption of it to the negatively charged silver particles. However, when silver nanoparticles are effectively modified with the iron(III)NTA complex, DA will be attracted to the iron center and be held there. Therefore, the presence of the Fe(NTA) complex on silver nanoparticles is beneficial in terms of intensity.

Table 1. Raman Shifts (cm−1) of Dopamine, Iron Dopamine, and Iron-NTA-Dopamine Complexes4,31−33,35 mode

Fe(NTA)DA

Fe(DA)2

υ15 υC−O υ3 υ19a υ19b υ8a

1150 1270 1326 1425 1480 1567

1151 1267 1325 1426 1483 1567

DA

1337 1470

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Figure 2. Schematic representation of various detection strategies: SERS measurements of DA and Fe(DA)2 complex utilizing bare AgNPs and SERS measurements of DA obtained with Fe(NTA) modified AgNPs as substrate. Figure numbers corresponding to their SERS spectra are provided together with UV−vis spectra of (a) DA molecules, (b) Fe(DA)2 aqueous complex, and (c) Fe(NTA)DA aqueous complex.

concentrations. The time dependent SERRS signals collected immediately and 15 min after the addition of 1 × 10−5 M DA solution onto Ag−Fe(NTA) substrate are the same (Figure 4). The whole SERRS assay, from DA binding to signal detection, lasted less than 1 min. The speed of the measurement confirmed that this SERS substrate has a great potential for high-throughput detection of DA for many applications. The Fe(NTA) coating method is a single-step procedure that can be performed very quickly. This advantage would be an important factor in any analysis depending on the degree of repeatability of substrate preparation. SERS substrates taken from the same batch and also from different batches of preparation were utilized for probing the repeatability of Fe(NTA) modified silver nanoparticles for DA detection. Sequential SERS measurements of 1 × 10−5 M and 1 × 10−8 M DA were performed for periods of 1−2 min and the peak intensity for the marker band at 1480 cm−1 was followed. The batch-to-batch signal variation or variation in the same batch was less than 7.0% for 20 measurements in the case of 1 × 10−5 M DA. The signal variation in the same batch was 9.0% for 10 measurements even at 1 × 10−8 M concentration of DA as in neural extracellular fluids. The consistency of the signal intensity indicated high repeatability of the measurements, which is a critical concern in any quantitative analysis. Ten of the SERRS spectra acquired are displayed in Figure 5.

Figure 3. Influence of the surface coverage of AgNPs with Fe(NTA) on the SERRS signal strength: (A) SERS signal of 0.5 M DA on bare AgNPs, (B) SERS signal of 1 × 10−4 M DA complexed with Fe3+ ion on bare AgNPs, and (C) SERRS signal of 1 × 10−5 M DA obtained with Fe(NTA) modified AgNPs.

Furthermore, the use of Ag−Fe(NTA) nanoparticles as SERS substrate permits fast probing of the DA at low 7732



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only 50%. The interference effect of AA on DA detection is a major problem when designing an electrochemical DA sensor. Since vibrational information is specific to the chemical structure of the molecules, spectral interference of AA is not expected in SERS measurements of DA.32,33 The typical concentration of AA in neural extracellular fluid is 1 × 10−4 M, whereas the DA concentration is in the submicromolar to micromolar range.2,40 Owing to the large difference in their concentrations (>102), the selectivity of the Ag−Fe(NTA) SERS substrate to DA was investigated at various AA concentrations that were 1 to 3 orders of magnitude higher than DA concentrations (Figure 6). The absence of any measurable differences in relative SERRS intensities of DA signals at or higher than physiological resting levels of AA was an indication of the excellent selectivity of the Ag−Fe(NTA) probe. Detection of DA at the resting level (0.01−1 μM) is of great clinical importance. The sample SERRS spectra of DA corresponding to concentrations between 0.5 × 10−9 to 4.0 × 10−9 M are given in Figure 7. The SERRS spectrum corresponding to the DA concentrations of 1 × 10−9 M (Figure 7C) is still fully recognizable at this low concentration in which sophisticated sample preparation steps are avoided. This result clearly demonstrates the detection potential of the Ag−Fe(NTA) SERRS probe for DA in most of the brain regions where DA is a major neurotransmitter. The detection limit of the measurement (3 s) was estimated to be 6.0 x10−11 M from Figure 7C. We deliberately carried out experiments at a low laser power (10 mW) and short data acquisition times (10 s), which might be considered realistic in a biomedical application.

Figure 4. Effect of the complexation time between Fe(NTA) modified silver nanoparticle and 1 × 10−5 M DA on the SERRS signal strength. SERRS signal of 1 × 10−5 M DA attained (A) immediately, (B) 15 min after injection of the solution on Fe(NTA)-modified silver nanoparticle substrate.

Ag is often a metal of choice for substrate preparation compared to its Au counterpart because of its inherently higher SERS activity. The common problem in the usage of silver nanoparticles as a plasmon source is the loss of sensitivity due to the oxidation of silver.39 The SERRS activity of Fe(NTA) coated silver particles stored in aqueous solution was investigated over 15 days and found to be stable for 1 week (Supporting Information, Figure S3). Even at the end of the second week their enhancement properties were diminished

Figure 5. Exemplary of SERRS spectra of 1 × 10−5 M DA by using Ag−Fe(NTA) nanoparticles prepared (a) in the same batch, (b) in different batches, and (c) SERRS spectra of 1 × 10−8 M DA by using Ag−Fe(NTA) nanoparticles prepared in the same batch. 7733



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The high sensitivity along with the improved selectivity of this sensing approach is a significant step toward molecular diagnosis of Parkinson’s disease. All of these properties mentioned above made the obtained substrate a perfect choice for routine SERRS detection application of DA. This nanotechnology-based method can be adapted for the detection of a wide variety of neurotransmitters.

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ASSOCIATED CONTENT

S Supporting Information *

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

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AUTHOR INFORMATION

Corresponding Author

*Fax: +903122103200. E-mail: [email protected]. Notes

Figure 6. SERRS spectrum of the (A) 1 × 10−5 M DA, (B) 1 × 10−5 M DA in 1 × 10−4 M ascorbic acid (AA), (C) 1 × 10−5 M DA in 1 × 10−3 M AA, (D) 1 × 10−6 M DA, (E) 1 × 10−6 M DA in 1 × 10−3 M AA, and (F) solid AA acquired with Ag−Fe(NTA) nanoparticles as substrate.

The authors declare no competing financial interest.

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REFERENCES

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Figure 7. SERRS spectrum of (A) 4 × 10−9 M, (B) 2 × 10−9 M, (C) 1 × 10−9 M, and (D) 0.5 × 10−9 M DA solution acquired with Ag− Fe(NTA) nanoparticles as substrate. Baseline correction was applied to all spectrum.

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CONCLUSION In brief, we have reported the development of a novel SERRS platform that allows fast and sensitive detection of DA molecules through surface-enhanced resonance spectroscopy. To the best of our knowledge, this is the first time that direct detection of a DA molecule attached to a Ag−Fe(NTA) by using SERRS has been achieved. The Ag−Fe(NTA) substrate provides reliable reproducibility, good time stability, and notable sensitivity. DA could be detected in pM level without any pretreatment with a reliable discrimination against AA. The pM concentration achieved in this study was among the lowest values reported for SERRS or SERS detection of DA. This SERRS assay takes less than 1 min from DA binding to detection and analysis. The experimental results reported herein represent a new possibility for the rapid, easy, and reliable diagnosis of DA by measuring the SERRS intensity. 7734



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New Approach for the Surface Enhanced Resonance Raman

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