SERRS Spectra and Excitation Profiles of Ru(II) Polypyridine

Article pubs.acs.org/JPCC

SERRS Spectra and Excitation Profiles of Ru(II) Polypyridine Complexes Attached to Ag Nanoparticle Aggregates: Structural, Electronic, and Resonance Damping Effects of Chemisorption Markéta Kokošková,*,† Marek Procházka,‡ Ivana Šloufová,† and Blanka Vlčková*,† †

Department of Physical and Macromolecular Chemistry, Charles University in Prague, Hlavova 8, Prague 2, 128 40, Czech Republic Institute of Physics, Charles University in Prague, Ke Karlovu 5, Prague 2, 121 16, Czech Republic

‡

S Supporting Information *

ABSTRACT: Surface enhanced resonance Raman scattering (SERRS) spectra and SERRS excitation profiles of a dicationic Ru(II) bis(2,2′-bipyridine)(4,4′-dicarboxy-2, 2′-bipyridine) (Ru(bpy)2(dcbpy)) complex in systems with Ag nanoparticle (NP) aggregates were obtained and compared to those of a dicationic Ru(II) tris(2,2′-bipyridine) (Ru(bpy)3). SERRS spectra provided evidence for chemisorption of Ru(bpy)2(dcbpy) complex onto Ag NP surface via at least one of the carboxylate groups. SERRS excitation profiles of the spectral bands attributed to the Ru-dcbpy unit of the chemisorbed Ru(bpy)2(dcbpy) complex were found to maximize at 488 nm excitation, while those of the two Ru-bpy units peaked at 458 nm. Comparison of the profiles with the electronic absorption spectrum of free Ru(bpy)2(dcbpy) indicates that chemisorption of the complex causes a red-shift of the Ru(II) → dcbpy MLCT (metal to ligand charge transfer) transition band. The energy decrease of the Ru(II) → dcbpy MLCT transition is explained by an increase of the electron-withdrawing ability of the two COO− groups upon their chemisorption on Ag NP surface. Finally, concentration values of SERRS spectral limits of detection of both complexes in system with uniform morphologies of Ag NP aggregates were determined, yielding the 1 × 10−9 M value for the chemisorbed Ru(bpy)2(dcbpy) and the 1 × 10−12 M value for the electrostatically bonded Ru(bpy)3. The major contribution to this difference is attributed to ca. 500× higher resonance damping in SERRS of the chemisorbed complex in comparison to that in SERRS of the electrostatically bonded one.

1. INTRODUCTION Surface-enhanced resonance Raman scattering (SERRS) of chromophoric molecules located at the surfaces of plasmonic metal nanostructures takes advantage from a combination of the surface and the (molecular) resonance enhancement of their Raman scattering which, in turn, is conditioned by selection of the excitation wavelength which obeys simultaneously the localized surface plasmon resonance (LSPR) condition of the particular plasmonic metal nanostructure and the molecular resonance condition of the particular chromophoric species located at its surface. SERRS spectroscopy thus combines fingerprint selectivity with tremendous (attomol or even zeptomol) sensitivity and has been developed into a valuable spectroanalytical tool with a wide range of applications.1−6 Despite the mature state of its applications, there are a few interesting aspects of SERRS, which appear to be worthy of a closer inspection, such as the conditions and consequences of the geometric and electronic structure perturbation of a chromophore upon its bonding to a particular plasmonic nanostructure surface. © 2012 American Chemical Society

Another point of interest in SERRS is a question whether or not SERRS of a chromophore located at a particular type of nanostructure can be viewed as a simple superposition of the electromagnetic (EM) mechanism of SERS and RRS (resonance Raman scattering) of a free molecule. In particular, it has been demonstrated that the SERRS enhancement experienced by Rhodamine 6G adsorbed on Ag island films is by 2 orders of magnitude lower that a multiplication of the RRS enhancement of free Rhodamine 6G by the EM mechanism enhancement, which stems from the proximity of the Ag island films.7 This effect has been attributed to shortening of the molecular excited state lifetime by opening of a new nonradiative deactivation channel toward the excited surface plasmon (SP) state, and it has been dubbed molecular resonance damping.7 In contrast to that, no damping has been found in SERRS of Fe(bpy)3 complex dication adsorbed Received: October 25, 2012 Revised: November 26, 2012 Published: December 18, 2012 1044

dx.doi.org/10.1021/jp310579j | J. Phys. Chem. C 2013, 117, 1044−1052

The Journal of Physical Chemistry C

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on isolated Ag NP of tetraedral shapes prepared by nanolithography.8 In addressing the issue of resonance damping, the authors in ref 8 actually considered a possibility of surface plasmon damping by the adsorbed complex rather that the molecular resonance damping as explained in ref 7. Interestingly, in the case of hybrid molecule−surface plasmon states formation described in ref 9 (as resulting from a strong coupling between surface plasmon state in hole arrays and S2 molecular excited state of J-aggregate of tetrasulphonatophenyl porphine molecules located in the holes), the individual surface plasmon (SP) and molecular excited states are no longer distinguished, yielding two coupled SP-S2 states (the lower of them with a very short 700 fs lifetime). Apparently, by using stationary SERRS and RRS spectral measurements, we are not able to distinguish between the three possible sources of the resonance damping outlined above. However, we are able to investigate the factors that affect the magnitude of the resonance damping in SERRS of a chromophore located within a particular plasmonic NP assembly. In this article, we focus our attention first on the effect of chromophore-Ag NP surface bonding, namely, on the effect of chemisorption of a selected chromophore on its electronic structure. Furthermore, we mutually compare the effect of chemisorption and of the electrostatic bonding of structurally analogous cationic chromophores to Ag NP surfaces (in their fractal aggregates) on the resonance damping in their SERRS spectra. As model chromophores, we selected Ru(II) polypyridine complexes, for which a wealth of information about their electronic structure in their free, i.e., nonadsorbed, state is available, particularly due to their utilization as sensitizers in dye-senzitized solar cells and as photocatalysts for splitting of water.10,11 In particular, resonance Raman (RR) spectroscopy has been demonstrated to be an important tool for elucidation of the electronic structure of heteroleptic Ru(II) polypyridine complexes and of its tuning by means of sidespecific substitutions of the ligands.11−14 The particular selected species are complex dications [Ru(bpy)3]2+ (denoted as Ru(bpy)3) and [Ru(bpy)2(dcbpy)]2+ (denoted as Ru(bpy)2(dcbpy), where bpy = 2,2′-bipyridine and dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) (Figure 1A,B). In systems with Ag NP hydrosols, Ru(bpy)3 dication is known to be electrostatically attached to negatively charged ions on the surface of Ag NP.15,16 For Ru(bpy)2(dcbpy), we tried to find the conditions under which it will be attached to Ag NP in a different manner than Ru(bpy)3, namely by chemisorption of one or both carboxylate groups of the dcbpy ligand. Toward this goal, we explored and compared SERRS spectra of both complexes adsorbed on Ag NP, which are weakly stabilized by adsorbed borates17 as well as on the surfaces of Ag NP modified by adsorption of chlorides,15,18 and we provide evidence that Ru(bpy)2(dcbpy) is chemisorbed on Ag NP surfaces weakly stabilized by adsorbed borates by at least one of the two carboxylate groups. Furthermore, we explore the effect of chemisorption onto the electronic structure of the heteroleptic Ru(bpy)2(dcbpy) complex by means of SERRS excitation profiles of the Ru-bpy and Ru-dcbpy unit spectral bands. Finally, we compare SERRS spectral limits of detection (LODs) of the two complexes in systems with fractal aggregates of Ag NP, evaluate the mutual ratio of their SERRS enhancement factors, and compare the estimated values of their resonance damping factors.

Figure 1. Structures of Ru(bpy)3 (A) and Ru(bpy)2(dcbpy) (B) and (C) their electronic absorption spectra in aqueous solutions (cM = 1 × 10−4 M). Excitation wavelengths used for SERRS spectral measurements of both complexes are marked by arrows.

2. EXPERIMENTAL SECTION 2.1. Materials. Analytical grade AgNO3, NaBH4, and NaCl were purchased from Merck and KClO4 from Penta. Tris(2,2′bipyridyl) ruthenium(II) chloride hexahydrate was obtained from Fluka and cis-bis(2,2′-bipyridyl)-(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) hexafluoro-phosphate (Ruthenizer 455-PF6) from Solaronix SA. Distilled deionized water was used for all sample preparations. 2.2. Preparation Procedures. Ag NP hydrosol was prepared by reduction of AgNO3 by NaBH4 in aqueous ambient using Procedure I described in ref 19. Ag NP hydrosol/complex systems (with unmodified Ag NPs) for SERRS spectral measurements were prepared by adding 20 μL of 1 × 10−4 M aqueous solution of the complex to 2 mL of Ag NP hydrosol yielding 1 × 10−6 M final concentration of the complex in the system. The samples were prepared one day prior to the spectral measurements and stirred for one hour and for three hours in the case of Ru(bpy)3 and Ru(bpy)2(dcbpy), respectively. AgNP hydrosol/complex/NaCl systems (with chloride-modified Ag NP) resulted from the addition of 100 μL of 1 M NaCl to Ag NP hydrosol/complex systems (prepared as above) immediately before the spectral measurements. The final concentration of chloride ions in the systems was 5 × 10−2 M. AgNP hydrosol/Ru(bpy)2(dcbpy)/EtOH systems for SERRS spectral measurements as a function of excitation wavelengths, targeted on construction of SERRS excitation profiles of Ru(bpy)2(dcbpy) spectral bands, were prepared by adding 100 μL of ethanol as an internal intensity standard into the Ag NP hydrosol/Ru(bpy)2(dcbpy) system (2.02 mL volume, prepared by the procedure described above) immediately before the spectral measurements. Addition of EtOH did not cause any Ag NP aggregation in the system. Ag NP hydrosol/Ru(II) complex/KClO4 systems (constituted by fractal aggregates of Ag NP) for SERRS spectral measurements 1045



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interfere with any spectral band of the samples and could clearly be distinguished in the spectra. The error caused by the difference in υ4 values of the photons scattered upon excitation of the 880 cm−1 band of ethanol and of the highest of the band in SERRS spectra of the complexes, namely, the 1559 cm−1 band of Ru(bpy), within the SERRS excitation profile (441.6− 532 nm) was estimated as 3%. 2.6. Acquisition and Treatment of RRS and NRS Spectra. RRS spectra of 1 × 10−4 M aqueous solutions of both Ru(II) complexes were obtained with 441.6 nm excitation (Figure S1 in the Supporting Information). The effect of scattered light reabsorption in a cuvette was reduced by the right angle geometry of the spectrometer. Spectral background in the spectra for both complexes was corrected and the intensities (in terms of band areas) of eight RRS spectral bands common to both complexes (namely, 668, 1028, 1174, 1275, 1318, 1490, 1562, and 1608 cm−1) were compared. The average value of Ru(bpy)2(dcbpy) band intensities (I) to those of Ru(bpy)3 (II) ratio was determined as IRRS(I)/IRRS(II) = 0.5. Nonresonant Raman spectra of the pure complexes in solid state were obtained with 784.8 nm excitation, and they are presented in Figure S2 in the Supporting Information. 2.7. Calculation of the Resonance Damping Factors Ratio of the Complexes. The mutual ratio of the resonance damping factors of Ru(bpy)2(dcbpy) (I) and of Ru(bpy)3 (II) was calculated by using the following formula:

as a function of the complex concentration were obtained by adding 20 μL of 1 × 10−6−1 × 10−11 M aqueous solution of the complex to 2 mL of Ag hydrosol and stirred and aged as described above. Approximately 15 min prior to spectral measurements, 200 μL of 0.1 M KClO4 were introduced into these systems as an aggregation agent20 to obtain aggregates of a uniform morphology. The final concentration of KClO4 in all systems was 1 × 10−2 M. Finally, 1 × 10−4 M aqueous solutions of Ru(bpy)3 and Ru(bpy)2(dcbpy) were employed for the RRS spectral measurements . 2.3. Instrumentation. SERRS and RRS spectra were recorded with a multichannel Raman spectrometer equipped with a monochromator Jobin-Yvon-Spex 270 M (1800 gr/mm grating) and liquid N2-cooled CCD detector Princeton Instruments (100 × 1340 pixels) in a right angle scattering geometry. Elimination of Rayleigh scattered light was provided by holographic notch-plus filter (Kaiser) located in front of the monochromator input slit. Five excitation wavelengths were used: 441.6 (He−Cd laser Liconix 4230N, 3 mW), 457.9, 488.0, 514.5 (argon laser Innova 300, ∼100 mW), and 532 nm (a diode-pumped frequency-doubled Nd:YVO4 laser, Verdi, Coherent, 100 mW). Nonresonant Raman (NRS) spectra of the complexes in solid state were measured with a DXR Raman microscope (Thermo Scientific) interfaced to an Olympus microscope. An objective with 10× magnification and highresolution grating were used. The excitation was provided by the 784.8 nm laser line (diode laser), with the laser power set to 10 mW. UV/vis absorption spectra were measured with an Analytik Jena Specord 2050 spectrometer. TEM images were obtained with a JEOL-JEM 200 CX transmission electron microscope. 2.4. Determination of Fractal Dimensions of Ag NP Aggregates. Fractal dimensions of Ag NP aggregates (D values) deposited from Ag NP hydrosol/complex/KClO4 systems were determined from their TEM images using the mass−radius relationship M ≈ RD (where M is the mass of the object of size R)21 and adopting a modification of the procedure reported in ref 22. An image analysis computer program (NIS-Elements AR3.20, Laboratory Imaging, Ltd.) was employed for evaluation of D values of the aggregates. Briefly, the image of an aggregate was covered by a set of concentric squares of regularly increasing sizes (√S) and areas S. The D value of an aggregate was obtained as a slope of the plot of ln A as a function of ln √S, where A is the area occupied by Ag NPs within the square of a total area S. The average values of fractal dimensions of aggregates deposited from Ag NP hydrosol/Ru(bpy)3/KClO4 and Ag NP hydrosol/Ru(bpy)2(dcbpy)/KClO4 systems have been identical yielding D = 1.89. 2.5. Acquisition of SERRS Spectra and Construction of SERRS Excitation Profiles. SERRS spectra of Ag NP hydrosol/Ru(bpy)2(dcbpy)/EtOH systems were measured at five excitation wavelengths (441.6, 457.9, 488.0, 514.5, and 532 nm), and then, the spectral backgrounds were adjusted by using the orthogonal differences method in combination with factor analysis. In order to plot SERRS excitation profiles (i.e., dependence of relative intensity band on excitation wavelength), it was necessary to obtain relative band intensity values. For this purpose, an internal intensity standard (ethanol) was employed. Areas of spectral bands in all five spectra were divided by the 880 cm−1 band area of EtOH. Spectral bands were normalized with respect to the 880 cm−1 band as the most intense and well-separated band of ethanol, which did not

ΓRu(bpy)2 (dcbpy) ΓRu(bpy)3 ≅ ×

IRRS(I) IRRS(II)

×

=

G RRS(I) GMR(II) ΓI = × G RRS(II) GMR(I) ΓII

GSERRS(II) GSERRS(I)

LODSERRS(I) LODSERRS(II)

×

×

G EM(I) G EM(II)

≅

IRRS(I) IRRS(II)

G EM(I) G EM(II)

In this formula, the resonance damping factor Γ is defined as Γ = GRRS/GMR where GRRS represents the resonance enhancement factor and GMR is the molecular resonance enhancement factor, which, in turn, is given as GMR = GSERRS/GEM. Furthermore, the GRRS(I)/GRRS(II) ratio has been approximated by the IRRS(I)/IRRS(II) determined experimentally in section 2.6. Finally, the ratios of the SERRS enhancement factors GSERRS(I)/ GSERRS(II) were approximated by the inverse ratio of LOD values of the two complexes I and II determined experimentally in section 3.3. In the same section, the arguments for the evaluation of GEM(I)/GEM(II) ≈ 1 are provided as well.

3. RESULTS AND DISCUSSION 3.1. SERRS Spectra of Ru(bpy)2(dcbpy) and Ru(bpy)3: Evidence of Ru(bpy)2(dcbpy) Chemisorption on Ag NP Surface. We have started our study with measurement and interpretation of SERRS spectra of the complexes obtained from Ag NP hydrosol/complex systems at 441.6 and 532 nm excitations. A projection of these excitation wavelengths into the electronic absorption spectra of the complexes (Figure 1C) demonstrates that the molecular resonance contribution to the overall, i.e., SERRS, enhancement is substantially larger at the 441.6 nm excitation than at the 532 nm one. SERRS spectrum of Ru(bpy)2(dcbpy) adsorbed on Ag NPs weakly stabilized by adsorbed borates (unmodified Ag NP surfaces) excited at 441.6 nm (Figure 2, spectrum a) shows several additional bands (namely, at 700, 1257, 1295, 1366, 1046



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a) indicates that, on the unmodified Ag NP surfaces, Ru(bpy)2(dcbpy) is adsorbed by coordination (chemisorption) of the carboxylate groups as depicted in Figure 3B. By contrast, upon the addition of chlorides, the carboxylate bonding is not strong enough to resist adsorption of chlorides and formation of chloride-modified Ag NPs.27 Ru(bpy)2(dcbpy) thus appears to be attached to chloride modified Ag NP surfaces by electrostatic interaction between the adsorbed chlorides and the dicationic complex, i.e., in the same manner as Ru(bpy)3 (Figure 3A). SERRS spectra of the complexes excited at 532 nm (Figure 4) are in a good agreement with the corresponding SERRS spectra excited at 441.6 nm (Figure 2) despite the difference in the molecular resonance contribution to SERRS at each of the two excitation wavelengths. In particular, doublets of close-lying spectral bands appear also in the SERRS spectrum of Ru(bpy)2(dcbpy) adsorbed directly on the unmodified Ag NP surfaces (Figure 4A). Within these doublets, their first components (namely, 1273, 1488, and 1559 cm−1) exactly match the spectral bands in SERS of Ru(bpy)3 (Figure 4B), and they are assigned to the Ru-bpy units. The second components (namely, 1256, 1474, and 1534 cm−1) are in a good agreement with the bands of the Ru-dcbpy unit in the RR spectra of the Ru(dcbpyH2)2(NCS)2 complex adsorbed on TiO2,23 and together with the 1365 cm−1 of chemisorbed carboxylates (Figure 4A), they are attributed to the Ru-dcbpy unit attached to Ag NP surface via the carboxylate groups. A detailed comparison of SERRS spectral bands of Ru(bpy)2(dcbpy) and Ru(bpy)3 is presented in Table 1, together with the RR spectral bands of the Ru-dcbpy unit23 and RR spectral bands of Ru(bpy)3 and their assignment based on NCA.28 3.2. SERRS Excitation Profiles of Ru(bpy)2(dcbpy) Spectral Bands. Furthermore, the excitation wavelength dependence of the SERRS spectra of the chemisorbed Ru(bpy)2(dcbpy) has been investigated in more detail. The spectra of the Ag NP hydrosol/Ru(bpy)2(dcbpy)/EtOH system were measured at five different excitation wavelengths and SERRS excitation profiles of spectral bands were constructed. Our attention was focused specifically onto the doublets of bands in which one component belongs to the Rubpy units and the other to the Ru-dcbpy unit attached to the Ag NP surface. In particular, SERRS excitation profiles of the 1488 and 1559 cm−1 bands of the Ru-bpy units and the 1474 and 1537 cm−1 bands of the Ru-dcbpy unit are shown in Figure 5A. Surface plasmon extinction (SPE) spectrum of the Ag NP hydrosol/Ru(bpy)2(dcbpy)/EtOH system employed in SERRS spectral measurements is shown as an inset into Figure 5A, while the electronic absorption spectrum of Ru(bpy)2(dcbpy) in aqueous solution together with the projections of the excitation wavelengths used for the SERRS measurements is presented in Figure 5B. First, we have observed that none of the excitation profiles (Figure 5A) matches the SPE spectrum of the Ag NP hydrosol/ Ru(bpy)2(dcbpy)/EtOH system (inset to Figure 5A). This result indicates that the SERRS excitation profiles are governed by the molecular resonance contribution to the overall enhancement. The dominant effect of molecular resonance over the SERRS excitation profiles has been reported earlier for Ru(bpy)3 in AgNP hydrosol/HCl/Ru(bpy)3 systems15 as well as for several other dyes adsorbed on Ag NP surfaces.6 Second, we have noticed the match between the excitation profiles of spectral bands belonging to the same structural unit (Figure

Figure 2. SERRS spectra of complexes in systems with Ag NP hydrosol excited at 441.6 nm. Ru(bpy)2(dcbpy) (a), Ru(bpy)2(dcbpy) after addition of NaCl (b), and Ru(bpy)3 (c).The SERRS spectrum of Ru(bpy)3 remained unchanged after the addition of NaCl.

1426, 1475, and 1537 cm−1) in comparison to that of Ru(bpy)3 obtained with the same excitation wavelength (Figure 2, spectrum c). While both the wavenumbers and relative intensities of bands in the SERRS spectrum of Ru(bpy)3 are the same for Ru(bpy)3 in the system with the unmodified and with the chloride-modified Ag NPs (Figure 2, spectrum c), no additional bands appear in the SERRS spectrum of Ru(bpy)2(dcbpy) in the system with chloride-modified Ag NPs (Figure 2, spectrum b). Observation of the clearly distinguished additional bands, which correspond to those reported for the Ru-dcbpy unit (Table 1 and ref 23) and, most importantly, detection of the 1365 cm−1 typical for bidentatelly coordinated carboxylates24−26 in SERRS of Ru(bpy)2(dcbpy) (Figure 2, spectrum 1047



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Table 1. Wavenumbers of Ru(bpy)2(dcbpy) and Ru(bpy)3 Bands Observed in SERRS Spectra and Their Assignment in Comparison to the Previously Published Data Ru(dcbpy)23

Ru(bpy)2(dcbpy) λexc = 441.6 nm

λexc = 532 nm

λexc = 415.4 nm

λexc = 441.6 nm

λexc = 532 nm

SERRS

SERRS

RR

SERRS

SERRS

RR 370 A1

668

379 422 454 474 491 554 643 667

665 700 731 765 1026 1063 1106 1173 1257 1270 1295 1317 1366 1426 1475 1487 1537 1559 1603 a

Ru(bpy)328

Ru(bpy)3

358 424 455 476 490 552 645 666 700 732 765 803 1025 1042 1066 1106 1172 1256 1273 1294 1317 1367 1420 1474 1488 1534 1559 1605

645 B2 668 A1

1027 1040 1064 1106 1173

732 766 806 1025 1041 1064 1109 1172

1272

1273

1276 A1

1317

1317

1320 A1

1488

1487

1491 A1

1559 1603

1558 1602

1563 A1 1608 A1

765

1102

NCA

766 A1 1028 A1 1043c 1067 A1 1176 A1

1260a 1290a 1367b 1430 1471a 1539a 1610

Characteristic bands for Ru(dcbpy) structural unit.23 bCoordinated carboxylate band.24 cNot assigned.

the lack of a direct contact between the Ru-bpy unit and Ag NP surface. In contrast to that, Ru-dcbpy unit band profiles maximize at 488.0 nm excitation, although this excitation wavelength falls only into the slope of the electronic absorption band of free Ru(bpy)2(dcbpy) complex (Figure 5B). In search for a possible explanation of this observation, we turned our attention to the difference between the electronic absorption spectrum of Ru(bpy)3 and Ru(bpy)2(dcbpy) (Figure 1C), namely, a small but noticeable extension of Ru(bpy)2(dcbpy) absorption band toward the red spectral region (at ca. 470 nm) in comparison to that of Ru(bpy)3. This barely visible shoulder is attributed to Ru(II) → dcbpy MLCT transition, which, in the free complex in aqueous solution (with both carboxylates dissociated to COO−), nearly overlaps with the Ru(II) → bpy MLCT transition.29,30 However, it has been reported previously that this electronic absorption band shifts markedly toward the red (namely, to 488 nm) upon protonization or esterification of the COO− groups of the dcbpy ligand.29,30 This red shift corresponding to a substantial decrease of the energy of the Ru(II) → dcbpy MLCT transition has been explained by an increase of the electron withdrawing ability of the carboxylate groups upon their protonization or esterification.29,30 Observation of the maximum of SERRS/SERS excitation profiles of the Ru-dcbpy unit bands at 488.0 nm indicates that a similar decrease of the Ru(II) → dcbpy MLCT transition most probably occurs also upon chemisorption of the Ru(bpy)2(dcbpy) complex onto AgNP surface. We propose that,

Figure 3. Schematic illustration of the different types of complex bonding onto the borate-stabilized Ag NP surface. Electrostatic interaction of Ru(bpy)3 (A) and chemisorption of Ru(bpy)2(dcbpy) (B).

5A). By contrast, the shapes and the maxima of the profiles plotted in Figure 5A are different for the spectral bands of each of the two structural units: while those of the Ru-bpy unit peak at 457.9 nm excitation, those of Ru-dcbpy unit maximize at 488.0 nm. The maximum on the profiles of the Rubpy unit bands corresponds very well with the maximum of the electronic absorption band in the spectrum of Ru(bpy)2(dcbpy) complex in aqueous solution (i.e., in its nonadsorbed state) positioned at 455 nm (Figure 5B) and assigned to Ru(II) → bpy MLCT (metal to ligand charge transfer) transition.29,30 Their correspondence indicates that the energy of the Ru(II) → bpy MLCT transition does not change noticeably upon chemisorption of the Ru(bpy)2(dcbpy) complex onto the Ag NP surface. The most probable reason is 1048



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Figure 5. SERRS excitation profiles of spectral bands (A) and SPE spectrum of Ag NP hydrosol/Ru(bpy)2(dcbpy)/EtOH (inset to A). Electronic absorption spectrum of Ru(bpy)2(dcbpy) aqueous solution (cM = 1 × 10−4 M). The excitation wavelengths used for SERRS measurements are marked by solid arrows; a shoulder assigned to Ru(II) → (dcbpy) MLCT transition is marked by a dashed arrow (B).

Figure 4. SERS spectra of Ru(bpy)2(dcbpy) (A) and Ru(bpy)3 (B) excited at 532 nm. Ru(dcbpy) structural unit bands are marked by dashed arrows; Ru(bpy) unit bands by full arrows. Inserts: Structures of the complexes with the Ru(dcbpy) unit marked by a dashed line and Ru(bpy) units by full lines. Spectrum B has been baseline corrected to remove the increasing emission background.

with the complex concentrations corresponding to their LODs are presented in Figure 7. The LOD values are 1 × 10−9 M for Ru(bpy)2(dcbpy) and 1 × 10−12 M for Ru(bpy)3. Their comparison indicates that the electrostatically bonded Ru(bpy)3 experiences by 3 orders of magnitude higher overall SERRS enhancement than the chemisorbed Ru(bpy) 2 (dcbpy) in systems with fractal aggregates (D = 1.89) at 441.6 nm excitation. Considering the EM mechanism contribution to SERRS of the two complexes to be roughly the same owing to the uniform morphology of fractal aggregates in both systems, we can ascribe the 3 orders of magnitude higher SERRS enhancement experienced by Ru(bpy)3 in comparison to Ru(bpy)2(dcbpy) to the 3 orders of magnitude higher molecular resonance mechanism contribution to the overall SERRS enhancement. While some contribution to this difference in molecular resonance enhancement can stem from the fact that free Ru(bpy)3 shows a higher absorption at 441.6 nm than Ru(bpy)2(dcbpy) (and, consequently, a larger resonance enhancement of Raman scattering at this excitation wavelength), the major contribution to the difference can be expected to originate from a difference in the molecular resonance damping, which, in turn, stems from a different type of the complex−Ag NP surface bonding. To separate the effect of the chromophore strength from the effect of the

in this case, the electron withdrawing ability of carboxylate group(s) is increased by their coordination to Ag NP surface. 3.3. Comparison of Resonance Damping in SERRS/ RRS of Chemisorbed Ru(bpy)2(dcbpy) and Electrostatically Bonded Ru(bpy)3: Evaluation of the Effect of the Chromophore Chemisorption. Finally, SERRS spectra of Ru(bpy)2(dcbpy) and Ru(bpy)3 were measured as a function of the complex concentration in Ag NP hydrosol/Ru(II) complex/KClO4 systems at 441.6 nm excitation. Concentration values of SERRS spectral limits of detection (LODs) of the chemisorbed Ru(bpy)2(dcbpy) and of the electrostatically bonded Ru(bpy)3 in the systems with the same Ag NP assemblies morphology, i.e., in fractal aggregates of D = 1.89, were determined and mutually compared. The uniformity of Ag NP aggregate morphologies manifests itself not only by the same value of D but also by the match of the shapes of the SPE curves of both systems (Figure 6), and it was achieved by using potassium perchlorate as the aggregation agent. SERRS spectra of Ag NP hydrosol/Ru(II) complex/KClO4 systems obtained 1049



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Figure 7. SERRS spectra of Ag NP hydrosol/Ru(II) complex/KClO4 systems at λexc = 441.6 nm. Ru(bpy)2(dcbpy), cM = 1 × 10−9 M (a). Ru(bpy)3, cM = 1 × 10−12 M (b).

Ag NP aggregates, i.e., in systems with hot spots. The presence of hot spots (i.e., of spatially localized, very strong optical fields after an optical excitation) in fractal aggregates has been predicted theoretically31−33 and proved experimentally.34 The presence or absence of hot spots in a particular Ag NP assembly can possibly be another factor, which can influence the resonance damping in SERRS of chromophoric molecules, and the experiments addressing this issue are currently in progress in our laboratory. Furthermore, the remarkably low 1 × 10−12 M value of LOD for Ru(bpy)3 in the system with fractal aggregates is actually not very surprising since nearly the same LOD value (ca. 10−12 M) has been reported for Ru(bpy)3 in the system with chloride-modified Ag NPs.16 Although the TEM images of the systems are have not been provided in ref 16, further studies have shown that the addition of chlorides in 10 mM concentration (used in ref 16) leads to the formation of compact aggregates of intergrown Ag NPs18 in which the presence of hot spots has been predicted35 and evidenced.36 The very low detection limits of Ru(bpy)3 in systems with hot spots thus appear to result from a combination of strong EM mechanism enhancement with the electrostatic bonding of the chromophore to Ag NP surface, which, in turn, minimizes the resonance damping effect. The electrostatic bonding of chromophores to Ag NP surface (via a thin anionic spacer) thus provides a double advantage over chromophore chemisorption: (i) preservation of the native electronic structure and (ii) decreasing the resonance damping by two or three orders of magnitude.

Figure 6. SPE (UV/vis) spectra and TEM images (nm scale) of Ag NPs/Ru(II) complex/KClO4 systems at concentrations corresponding to SERRS spectral detection limits of the complexes. Ru(bpy)2(dcbpy), cM = 1 × 10−9 M; optical length of a cuvette, l = 1 mm (A). Ru(bpy)3, cM = 1 × 10−12 M; optical length of a cuvette, l = 2 mm (B).

chromophore bonding, we determined experimentally the average value of mutual ratios of the RRS band intensities at 441.6 nm, and we found out (by a procedure described in section 2.6) that this value is only two times higher for Ru(bpy)3 (II) than for Ru(bpy)2(dcbpy) (I), i.e., IRRS(I)/IRRS(II) equals 0.5. Taking into account that the mutual ratio of the SERRS spectral detection limits of the complexes, LOD (I)/ LOD (II) is 1 × 103, and inserting these experimentally determined values into the simple formula presented in the section 2.7 (while assuming that GEM(I) = GEM(II)), we obtain a roughly 500 times higher molecular resonance damping for a chromophore chemisorbed to Ag NPs in fractal aggregates than for an electrostatically bonded one. Nevertheless, taking into account the experimental precision of LODs and RRS intensities ratio determination, it is more prudent to set the factor by which the molecular resonance damping of the chemisorbed chromophore exceeds that of an electrostatically bonded one (in fractal aggregates of Ag NPs) into the 1 × 102− 1 × 103 interval. Furthermore, we would like to emphasize that our results were obtained for chromophores located in fractal 1050



4. CONCLUSIONS SERRS spectra of the dicationic Ru(bpy)2(dcbpy) complex in systems with aggregates of Ag NPs weakly stabilized by adsorbed borates (dubbed unmodified Ag NPs) have shown a distinct separation of the spectral bands of the Ru-bpy and Rudcbpy structural units as well as a new band at 1367 cm−1 assigned to υs (COO−) of carboxylate bidentatelly coordinated to Ag+ adsorption sites. These spectral features provide evidence of chemisorption of the complex onto Ag NP surface via at least one of the carboxylate groups. In contrast to that, in systems with Ag NPs modified by adsorbed chlorides, dicationic Ru(bpy)2(dcbpy) is adsorbed in the same manner as the Ru(bpy)3 dication, i.e., by the electrostatic interaction between the positively charged complex dication and the negatively charged surface of chloride-modified Ag NPs. Furthermore, SERRS excitation profiles of the spectral bands of Ru(bpy)2(dcbpy) chemisorbed onto unmodified Ag NPs show that the bands attributed to the Ru-dcbpy unit maximize at 488 nm excitation, while those of the two Ru-bpy units peak at 458 nm. Comparison of the profiles with the electronic absorption spectrum of free Ru(bpy)2(dcbpy) indicates that chemisorption of the complex causes a red-shift of the Ru(II) → dcbpy MLCT transition band. The observed energy decrease of the Ru(II) → dcbpy MLCT transition is explained by an increase of the electron-withdrawing ability of the COO− group upon its chemisorption on Ag NP surface. These results also demonstrate the importance of SERRS excitation profiles for obtaining information about the perturbation of the electronic structure of a chromophore upon its chemisorption onto Ag NP surface. In systems with plasmonic NPs, such information cannot be obtained by electronic absorption spectral measurements since, in the majority of cases, the absorption of a monolayer of the chromophoric molecules (or ions) is very weak and, moreover, masked by surface plasmon extinction of the plasmonic NP assembly. Finally, the concentration values of SERRS spectral limits of detection (LODs) of Ru(bpy)2(dcbpy) and Ru(bpy)3 in system with uniform morphologies of Ag NP aggregates are markedly different, yielding the 1 × 10−9 M value for the chemisorbed Ru(bpy)2(dcbpy) and the 1 × 10−12 M value for the electrostatically bonded Ru(bpy)3. The major contribution to this difference is attributed to a roughly 500× higher resonance damping in SERRS of the chemisorbed complex in comparison to that in SERRS of the electrostatically bonded one. Electrostatic bonding of cationic chromophores to negatively charged Ag NP surfaces is thus more advantageous than their chemisorption since it results in markedly lower LOD values of the chromophores, while, simultaneously, it preserves their native electronic structure.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Mrs. Jirina Hromadkova for her excellent technical assistance. Financial support by the P208/10/0941 grant awarded by the Czech Science Foundation and by the MSM 0021620857 long term research project awarded by MSMT CR is gratefully acknowledged.

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ASSOCIATED CONTENT

S Supporting Information *

Resonance Raman spectra of the complexes and solid-state Raman spectra of the complexes. 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] (B.V.); marketakokoskova@seznam. cz (M.K.). Notes

The authors declare no competing financial interest. 1051



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SERRS Spectra and Excitation Profiles of Ru(II) Polypyridine

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