Raman and XPS study on the interaction of taurine with silver nanoparticles

Abstract

This is the first report on the surface-enhanced Raman scattering (SERS) and X-ray photoelectron spectroscopic (XPS) studies of taurine adsorbed on the surface of silver nanoparticles. Taurine, the sulphonic acid analogue of β-alanine participates in numerous biological and physiological functions in the human body and is a potent antioxidant. The interpretation of the SERS spectrum based on the “surface selection rules”, XPS studies and DFT calculations has clearly indicated that the gauche tautomer of taurine is predominant on the silver nanoparticle surface. Taurine is directly bound to the silver surface through the sulphonate group along with interactions from the NH₂ group that lies in close proximity to the metal surface. The binding of taurine through the oxygen atoms of the sulphonate group is confirmed by the XPS studies which show the presence of 71% Ag–O in taurine functionalized silver nanoparticles.

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1. Introduction

The surface-enhanced Raman scattering (SERS) phenomenon is known to be governed by two mechanisms namely electromagnetic and chemical wherein the Raman signal of molecules attached to ‘rough’ metal surfaces or nanometer sized metallic colloidal particles gets enhanced and under favourable conditions the enhancement goes down to the single molecule level.^1–6 Apart from the high sensitivity of SERS, it serves as a probe to understand the interaction between the adsorbed molecules and the metallic surface. It is helpful in understanding the binding characteristics and the probable orientation of the molecule with respect to the metal surface using “surface-selection rules”^7–11 based on the image dipole field theory.¹² In our earlier SERS studies of various molecules we have investigated the binding characteristics and the probable orientation of the adsorbates on the metal surface.^13–22

Taurine (2-aminoethane sulphonic acid), the sulphonic acid analogue of β-alanine participates in numerous biological and physiological functions in the human body and is a potent antioxidant.^23,24 Apart from the reports on biological studies of taurine,^24–26 few reports on the vibrational analysis^27–29 and studies on the interaction of metal ions with taurine^23,30–32 are available in the literature. In this paper, we report the first SERS studies of taurine in silver nanoparticles and the probable orientation that the molecule assumes on the metal surface. XPS studies are also carried out to quantitatively understand the interaction of the molecule with silver nanoparticles. The interpretation of SERS spectrum is aided by DFT calculation and the application of “surface selection rules”.

2. Materials and methods

2.1 Chemicals

Taurine, silver nitrate and sodium borohydride were purchased from Sigma Aldrich. All the chemicals were used without further purification. All the experiments were performed with Millipore water (with resistivity of 18.2 MΩ cm at 25 °C).

2.2 Synthesis of silver nanoparticles (Ag NPs)

Ag NPs were prepared by the reduction of silver nitrate with sodium borohydride using the method of Creighton et al.³³ Freshly prepared NPs were used in the studies. The aggregation of NPs induced by the adsorbed molecules of taurine caused a change in color of the sol from yellow to pale pink and the process took nearly 30 min.

2.3 Instrumental details

Ag NPs and the taurine functionalized Ag NPs was characterized using UV-visible absorption spectrophotometer (JASCO V-650). The Raman spectra of taurine (solid and saturated aqueous solution) and taurine in silver sol were recorded at room temperature using the 532 nm line, from a DPSS Nd³⁺:YAG laser (Cobolt Samba 0532-01-0500-500) M/s Cobolt AB, Sweden. The laser power used to record the Raman spectrum was 25 mW and the spot size on the sample was ∼50 μm. For the Raman measurements, the sample solutions were taken in a standard 1 × 1 cm² quartz cuvette and the Raman scattered light was collected at 90° scattering geometry and detected using a CCD (Synapse, Horiba Jobin Yvon) based monochromator (Triax550, Horiba Jobin Yvon, France) together with a notch filter, covering a spectral range of 200–1700 cm⁻¹. X-ray photoelectron spectroscopy (XPS) measurements were carried out using Mg-K_α (1253.6 eV) source and DESA-150 electron analyzer (Staib Instruments, Germany). For the XPS analysis, the sample film was prepared by drop coating the sample on silicon (111) substrate and dried under the IR lamp. The binding-energy scale in XPS was calibrated to C 1s line of 284.8 eV.

3. Computational methods

To interpret the normal Raman and the SERS spectrum and to get more insight into the nature of binding of taurine with the Ag NPs and also to know the kind of orientation the molecule assumes with respect to the surface, the optimized geometry of taurine and the silver–taurine system was theoretically calculated. Geometry optimization was carried out using density functional theoretical (DFT) calculations for different tautomeric forms of taurine (gauche and trans as shown in Scheme 1) and their possible neutral and charged Ag as well as Ag₄ complexes with B3LYP functional³⁴ and aug-cc-pvdz (for taurine) and LANL2DZ (for Ag) basis sets using Gaussian 03 program.³⁵ No symmetry restriction was applied during geometry optimization. The vibrational frequencies for the two tautomeric forms of taurine and their neutral and charged Ag as well as Ag₄ complexes were computed at their optimized geometries to ensure that they correspond to local minimum on the potential energy surface and not to saddle points. The computed vibrations at the optimized geometries were then compared with the normal Raman and the SERS spectra.

Scheme 1 The gauche (I) and trans (II) tautomers of taurine.

4. Result and discussions

4.1 UV-visible absorption study

The absorption spectrum for the Ag NPs and the concentration-dependent taurine functionalized Ag NPs is shown in Fig. 1. The figure shows the surface plasmon resonance (SPR) band of Ag NPs at 378 nm. The absorption spectrum for different concentrations of taurine added to Ag NPs (final concentrations; 5 × 10⁻³ to 2.5 × 10⁻⁴ M), resulted in the decrease of absorbance at 378 nm followed by an increase in the overall absorbance in the higher wavelength range from 500–700 nm. The increase in absorbance in the longer wavelength region is possibly due to the colloidal aggregation^36,37 induced by adsorption of taurine over Ag NPs.

Fig. 1 The surface plasmon resonance (SPR) band of Ag NPs and the concentration-dependent UV-visible absorption spectrum of taurine functionalized Ag NPs.

4.2 Raman, SERS and DFT study

The normal Raman spectra of taurine, both in crystalline state and in aqueous solution (saturated) are shown in Fig. 2(a) and (b). Taurine is present in the zwitterionic form (NH₃⁺CH₂CH₂SO₃⁻) both in the solid and in the aqueous solution. The pK_a values of taurine are 1.5 and 8.74.³⁸ In solution, like amino acids, taurine can exist in cationic, zwitterionic and anionic species.²⁷ The Raman peaks are assigned to the various vibrations based on our theoretical results as well as referring to the earlier reported assignment.^27,28 Vibrations associated with CH₂, NH₃⁺ and SO₃⁻ is observed in the spectra and are tabulated in Table 1. The strongest peak observed at 1031 cm⁻¹ in the Raman spectrum of solid corresponds to the SO₃ symmetric (sym.) stretch (str.). The medium intense bands present at 530, 735 and, 1050 cm⁻¹ correspond to SO₃ sym. deformation (def.), CS str. and CN str., respectively. NH₃ def. modes appear as weak bands at 1588 and 1613 cm⁻¹. The presence of the 735 cm⁻¹ band that is assigned to the CS str. arises due to the gauche form of taurine. The Raman spectrum of solution shows a strong peak at 1045 cm⁻¹ corresponding to SO₃ sym. str. In addition to the peak observed at 742 cm⁻¹ that is assigned to the CS str. of the gauche form, a new peak is observed at 802 cm⁻¹ attributed to the CS str. of the trans form. Most of the other features as shown in Fig. 2(b) and Table 1 are weak with poor S/N ratio. The NH₃ sci. mode is not discernible and is probably masked by the H₂O def. mode. Our experimental observations are consistent with the literature data where taurine is known to exist in gauche form in the solid state while both gauche and trans forms co-exist in the aqueous solution.²⁷

Fig. 2 Normal Raman spectrum of taurine (a) solid (b) aqueous solution (saturated) and (c) SERS spectrum (5 × 10⁻⁴ M taurine in silver sol).

Table 1 Assignment of the Raman vibrations of taurine (solid and saturated solution) and taurine functionalized Ag NPs^a

Solid	Solution	Assignments	SERS	Assignments
a w: weak, m: medium, s: strong, vs: very strong, str.: stretch, sym.: symmetric, asym.: asymmetric, sci: scissoring, def: deformation.
			221m	AgO
232w		CCS bend
326w		SO3 rock, CCN bend
368m	355w	SO3 rock
471m	464w	CCN bend, SO3 sym. def.
530s	530w	SO3 sym. def.
593w	590w	SO3 asym. def.
735s	742w	CS str. (gauche)	798w	CS str. (gauche)
	802w	CS str. (trans)
846m	838w	CH2 rock, CN str.
895m	895w	CH2 rock
963w	958w	CH2 rock, CC str.	946w	CH2 rock, CC str.
1031vs	1045s	SO3 sym. str.	1008w	SO3 sym. str.
1050m		CN str.	1047w	CN str.
1110w	1110w	NH3 rock	1115w	NH2 rock
1181w		SO3 asym. str.	1153w	SO3 asym. str.
1219w	1200w	CH2 twist, SO3 asym. str.	1226w	CH2 twist, SO3 asym. str.
1259m		CH2 twist, SO3 asym. def.	1271w	CH2 twist, SO3 asym. def.
1344w	1334w	CH2 wag	1351m	CH2 def.
1427m, 1459m	1420w, 1461w	CH2 sci.	1386m	CH2 def.
1588w, 1613w		NH3 def.
	1630w	H2O def.
			1635s	NH2 sci.

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For the SERS measurements, solutions of different concentrations of taurine was mixed with the freshly prepared silver sol and a waiting time of 30 min was given before recording the spectrum [Fig. 2(c), 3 and 4]. The taurine functionalized Ag NPs were found to be stable for several days. The SERS spectrum obtained is considerably different from the normal Raman spectra of taurine with changes observed in the relative intensities of the corresponding Raman vibrations. In the silver sol at pH ∼ 9, taurine is present as anionic species, NH₂CH₂CH₂SO₃⁻. The strong Raman peak obtained at 1030 cm⁻¹, attributed to the SO₃⁻ sym. str., in the normal Raman spectrum of solid of taurine is seen as weak band at 1008 cm⁻¹ in the SERS spectrum while the weaker NH₂ def. band stands strong at 1635 cm⁻¹. A new peak at 221 cm⁻¹ assigned to the Ag–O str. is observed in the SERS spectrum which gives evidence for the chemical binding of the molecule with the silver surface through the oxygen atoms of the sulphonate group. The weak CH₂ def. bands observed in the normal Raman spectrum is seen as medium intense bands in the SERS spectrum. The SERS measurements were carried out for various pH ranging from 3.5 to 10.5 as shown in Fig. 3. The pH-dependent SERS studies show that the spectral features remain almost similar implying that the binding characteristics of taurine on Ag NPs remains almost the same in the mentioned pH range. At pH lower than 3.5, the Ag NPs get agglomerated and precipitated out.

Fig. 3 pH-dependent SERS spectra of taurine (5 × 10⁻³ M in silver sol).

Fig. 4 SERS spectra of taurine (a) 5 × 10⁻³ M, (b) 2.5 × 10⁻³ M, (c) 5 × 10⁻⁴ M and (d) 2.5 × 10⁻⁴ M in silver sol.

Unlike amino acids, taurine is known to form less stable complexes with various transition metal ions because the sulphonate group as compared to the carboxylate group (amino acids) is a poor ligand.³² To rule out the possibility of the presence of any photochemically transformed products of taurine over Ag NPs, the concentration-dependent SERS studies (Fig. 4) were carried out. SERS measurements were carried out for a small range of taurine concentration (2.5 × 10⁻⁴ M to 5 × 10⁻³ M). Poor signal was observed for concentrations lower than 2.5 × 10⁻⁴ M. Although for the taurine concentration of 2.5 × 10⁻⁴ M in silver sol, the SERS intensity was relatively low, it gained intensity and remained almost the same for higher concentrations (5 × 10⁻⁴ M, 2.5 × 10⁻³ M and 5 × 10⁻³ M). This behaviour suggests that monolayer coverage was achieved with 5 × 10⁻⁴ M concentration. With further increase in concentrations of taurine, the SERS intensities remained almost the same. This indicates that the SERS intensities for all the concentrations arise mainly from the first layer of taurine bound to Ag NPs. This also implies that the enhancement is mainly due to the “chemical effect” of SERS.

Further, the SERS spectra more or less resemble each other except in the CH₂ def modes which show some changes in the relative intensities of the two peaks around 1300 cm⁻¹. Thus, in the concentration range studied, there was also no indication of any photochemical change. The observed changes may be attributed to a slight change in the orientation of taurine involving the CH₂ groups, with the sulphonate and the amino groups remaining close to the metal surface.

4.3 XPS study

In order to quantitatively understand the binding affinity of taurine towards the Ag NPs, XPS analysis was carried out. The percentage composition of Ag, O, N and S element in taurine functionalized Ag NPs could be determined from the XPS study. From the XPS data, the surface composition (C_i) and the percentage of Ag–O, Ag–N and Au–S was calculated using the relation (1)^39,40where i, j = Ag, O, S and N. Here I_i represents the intensity of the Ag 3d, O 1s, N 1s and S 2p peaks and is determined by finding the total area under the core level peak using the least-squares fitting of Gaussian line shape. S_i is the atomic sensitivity factor and has values of 5.987, 0.711, 0.477 and 0.666 for Ag 3d, O 1s, N 1s and S 2p peaks, respectively. The values of S_i are standard and were taken from the literature.⁴¹ The XPS spectrum of Ag 3d, O 1s, N 1s and S 2p in taurine functionalized Ag NPs are shown in Fig. 5. The deconvoluted peaks of Ag 3d spectrum [Fig. 5(A)] show the peaks at binding energies of 366.7, 368.0, 372.8 and 374.3 eV. Among these, the peaks at 366.7 and 372.8 eV correspond to the oxide (Ag–O) and the peaks at 368.0 and 374.3 eV correspond to unbound Ag. Formation of Ag–O was also confirmed from the O 1s spectrum shown in Fig. 5(B) which shows three peaks at 528.3, 531.0 and 532.4 eV. The first peak at 528.3 eV was assigned to Ag–O, the strong peak at 531.0 eV corresponds to the O atoms from a sulphonate function,⁴² while the shoulder at 532.4 eV is attributed to the S=O group.⁴³ Formation of slight Ag–N was indicated from the N 1s spectrum as shown in Fig. 5(C) which clearly indicates the existence of nitride at 399.0 eV and the presence of C–NH₂ at 400.3 eV. The occurrence of little Ag–S was also established from the deconvoluted S 2p peak shown in Fig. 5(D) that contains four peaks at 163.0, 164.0, 168.4 and 169.5 eV. Of the four peaks, the former two correspond to sulphide (Ag–S) and the latter two peaks are attributed to the S 2p_3/2 and S 2p_1/2. The XPS analysis using relation (1), confirmed the presence of unbound Ag, Ag–O, Ag–N and Ag–S. XPS analysis clearly indicated ∼2% Ag was in unbound i.e. in the elemental form. Moreover, the percentage of Ag–O, Ag–N and Ag–S were found to be ∼71, 11 and 16, respectively. The XPS study thus, clearly indicated the abundance of Ag–O over Ag–S or Ag–N in taurine functionalized Ag NPs. This implies that the sulphonate group of taurine seems to be bound to the metal surface.

Fig. 5 XPS spectra of (A) Ag 3d, (B) O 1s, (C) N 1s and (D) S 2p peak of the taurine functionalized Ag NPs. Circle represents data points; black line is the fitting curve. Green, pink, red and brown lines are the deconvoluted peaks.

4.4 Optimized geometries of taurine and silver–taurine system and application of “surface selection rules”

The optimized structures of the gauche and trans conformers of taurine optimized using B3LYP functional and aug-cc-pVDZ basis set are shown in Fig. 6(A) and (B). Of the two tautomeric forms, it was observed that the gauche form was more stable (3.67 kcal mol⁻¹) as compared to the trans form. Although, mostly the Ag NPs are neutral in charge but some unreacted Ag⁺ may also remain on the surface which can interact with taurine. Hence, all possible structures of neutral and charged Ag complexes of taurine were optimized using B3LYP functional and aug-cc-pVDZ basis set for taurine and LANL2DZ for Ag and their optimized structures are given as Fig. S1(A)–(D) and S2(A)–(C) in ESI.† Among the possible calculated structures, the ones involving direct interaction of the sulphonate group with Ag were chosen and their geometries for the neutral and charged Ag₄ complexes were optimized. The optimized structures of gauche and trans conformers of neutral Ag₄ taurine complexes are shown in Fig. 7(A) and (B) and the charged Ag₄ taurine complexes are shown in Fig. 7(C) and (D). The Raman vibrations were computed at the optimized geometries for both conformations of gauche and trans taurine as well as neural and charged Ag₄ taurine complexes. The computed Raman spectra at the optimized geometries were then compared with the experimental Raman and SERS spectra. The optimized structures of the neutral and charged Ag₄ complexes show that in the gauche form along with the sulphonate group, the amino group was also in close proximity to the Ag₄ complex while in the trans form, the amino group was away from the metal surface.

Fig. 6 The optimized structures of (A) gauche, (B) trans conformer of taurine. The colour code used to identify the atoms are N (blue), O (red), S (yellow), C (grey) and H (white).

Fig. 7 The optimized structures of the neutral taurine–Ag₄ complex (A) gauche, (B) trans form and the charged taurine–Ag₄ complex (C) gauche and (D) trans form. The colour code used to identify the atoms are N (blue), O (red), S (yellow), C (grey), H (white) and Ag (light blue).

In order to find the most feasible structure among the four possible calculated geometries (Fig. 7) of taurine bound to the Ag NPs surface, the SERS spectra was analyzed and the “surface selection rules” invoked. This leads to the understanding of the sites of binding, the proximity of the various anchoring groups and hence, the probable molecular orientation of the adsorbate on the metal surface. Although, XPS results have clearly indicated the binding of taurine through the oxygen atoms of the sulphonate group, the SERS spectrum shows weak enhancement for the sym str. vibration of the sulphonate which is otherwise expected to be intense due to its direct binding to the surface. The normal modes of adsorbed molecules involving changes in molecular polarizability with a component perpendicular to the surface are subject to the greatest enhancement while the component parallel to the surface remains unaffected according to the “surface selection rules”. In all the four geometries [Fig. 7(A)–(D)], although the sulphonate group of taurine interacts with silver, the sym str. of the sulphonate group lie parallel to the surface and such vibrations according to the “surface selection rules” show weak or no enhancement in the SERS spectra though they are directly bound to the metal surface. In the SERS spectra, a prominent band is observed at 1636 cm⁻¹ attributed to the NH₂ def mode which suggests that the lone pair of the N atom of the NH₂ group also interacts with the metal surface. This is possible only if the NH₂ group is close to the surface. The gauche forms of taurine [Fig. 7(A) and (C)] has NH₂ group lying close to the surface as compared to the trans forms [Fig. 7(B) and (D)]. It is to be noted that among the two gauche forms Fig. 7(A) and (C), the NH₂ group in the charged Ag₄–taurine complex [Fig. 7(C)] is closer to Ag than in the neutral Ag₄–taurine form [Fig. 7(A)]. The proximity of NH₂ group in the gauche taurine allows the amino group to interact with the surface which explains the enhancement of NH₂ def band.

The SERS, XPS and DFT studies have clearly indicated that the gauche tautomer of taurine is predominant on the silver surface with the oxygen atoms of the sulphonate group being directly involved in binding. The proximity of the amino group allows the interaction of the lone pair of electrons on the nitrogen atom with the metal surface.

5. Conclusions

In this article, the SERS data in addition to the XPS and DFT results of taurine functionalized Ag NPs are reported for the first time. The binding of taurine through the oxygen atoms of the sulphonate group was confirmed by the XPS analysis which clearly indicated the presence of 71% Ag–O. A new peak at 221 cm⁻¹ assigned to the Ag–O str. is observed in the SERS spectrum which gives evidence for direct binding of the sulphonate group to the metal surface. The interpretation of the SERS data invoking the “surface selection rules” along with the calculated geometries suggests the predominance of the gauche conformer of taurine on the silver surface. Here, the NH₂ group that lies close to the metal surface interacts with the surface through the lone pair of electrons on the N atom and results in enhancement of the NH₂ def. band.

Acknowledgements

The authors thank Prof. K. Maiti of TIFR, Mumbai for the Gaussian 03W calculations.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The optimized structures of all possible neutral Ag–taurine and charged Ag–taurine complexes. See DOI: 10.1039/c6ra09569k

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