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Probing Formation, Structure and Reactivity of Zn(II), Ag(I) and Fe(II) Complexes with 2,2’:6’,2”-Terpyridine on Ag Nanoparticles Surfaces by Time-Evolution of SERS Spectra, Factor Analysis and DFT Calculations Ivana Sloufova, Blanka Vlckova, Peter Mojzes, Irena Matulkova, Ivana Cisarova, Marek Prochazka, and Jiri Vohlidal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12157 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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The Journal of Physical Chemistry

Probing Formation, Structure and Reactivity of Zn(II), Ag(I) and Fe(II) Complexes with 2,2’:6’,2”-terpyridine on Ag Nanoparticles Surfaces by TimeEvolution of SERS Spectra, Factor Analysis and DFT Calculations Ivana Šloufová1*, Blanka Vlčková1*, Peter Mojzeš2, Irena Matulková3, Ivana Císařová3, Marek Procházka2, Jiří Vohlídal1 1

Charles University, Faculty of Science, Department of Physical and Macromolecular Chemistry, Hlavova 2030, 128 40 Prague 2, Czech Republic

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Charles University, Faculty of Mathematics and Physics, Institute of Physics, Ke Karlovu 5, Prague 2, 121 16, Czech Republic

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Charles University, Faculty of Science, Department of Inorganic Chemistry, Hlavova 2030, 128 40 Prague 2, Czech Republic *

E-mail for Ivana Šloufová: [email protected]; Blanka Vlčková: [email protected]

ABSTRACT Adsorption and surface reactivity of 2,2’:6’,2”-terpyridine (tpy) and its metal (M = Zn2+, Fe2+, Ag+) complexes on Ag nanoparticle surfaces together with the structure of the resulting Msx+ -tpy surface species and the mechanism of their formation have been investigated. The process was elucidated by time-dependent surface-enhanced Raman scattering (SERS) spectra, their treatment by factor analysis (FA), density functional theory (DFT) calculations of Raman spectra and comparison of the SERS spectra of the surface species (obtained by FA) with the Raman spectra of their synthetic analogues (characterized by X-ray diffraction). SERS spectral timeevolution revealed surface-induced decomposition of [Zn(tpy)2]2+ complex and formation of + Zn2+ s -tpy and Ags -tpy surface species, accompanied by a ligand exchange of one tpy ligand for chloride anions on Zn2+ center. The inhibition of Ag+s -tpy surface species formation was achieved by addition of Zn2+ cations prior to the addition of [Zn(tpy)2]2+ complex. [Fe(tpy)2]2+ complex exhibited no surface decomposition and was identical to Fe2+ s -tpy surface species. x+ Additionally, generation of Ms -tpy species on the Ag nanoparticle surfaces by reaction of tpy and Mx+ cations was monitored by SERS and evaluated by FA. Determination of the Zn2+ s -tpy and + Ags -tpy structures was supported by DFT calculations of Raman spectra of Zn-tpy and Ag-tpy fragments.

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INTRODUCTION

Investigation of surface chemistry undergone by molecules on noble (currently known as plasmonic) metal surfaces has been a primary as well as persistent motivation for development of surface-enhanced Raman spectroscopy and of its widespread applications.1–6 A large progress in SERS spectral studies of dynamic processes on plasmonic nanoparticles and other nanostructured surfaces, has been achieved by employment of multivariate statistical methods, factor analysis (FA) in particular7, for evaluation of spectral sets of temporally evolving SERS spectra of a variety of surface species. For example, revealing and evaluation of the process of free base porphyrins metallation was of paramount importance for development of the strategies for their reliable SERS spectra detection and determination.8,9 FA also proved itself to be an invaluable tool for obtaining evidence of the presence of the common cationic Ag+ and the specifically generated neutral Ag(0) adsorption sites on Ag NP surfaces.10–12 The free base porphyrins (as tetradentate N-base ligands and adsorbates)10 as well as the bidentate 2,2'bipyridine11 (bpy) and, recently, also the tridentate 2,2’:6’,2”-terpyridine12 (tpy) were found to stabilize both the cationic and the neutral adsorption sites, and to form the spectrally different surface species at each of those. Transition metal complexes with the last mentioned, i.e. the tpy ligands, are currently the subject of considerable interest stemming from their prospective applications as active materials in dye sensitized solar cells, light emitting diodes as well as the building blocks13 of metallosupramolecular polymers and dynamers.14 In conjunction with these research interests, the solid state crystal structures as well as the kinetics of formation of the complexes of tpy with various central metal atoms were investigated.1,13,15–21 For example, both the Fe2+ and the Zn2+ cations were found to form pseudooctahedral bis(tpy) complexes.22–24 On the other hand, a comparative study of the kinetics of formation of these complexes by UV/vis and NMR spectral monitoring was published.18 Tpy ligand complexation in acetonitrile by a gradual addition of the Fe2+ and/or Zn2+ cations, respectively, has shown that, in the former case, there was a strong preference of the bis(tpy), i.e. the [Fe(tpy)2]2+ complex formation, whereas two steps with the nearly equal binding constants were found in the kinetics of the [Zn(tpy)2]2+complex formation.18 This result suggests a possible kinetic lability of the [Zn(tpy)2]2+complex under certain conditions. The synthetically prepared [Fe(tpy)2]2+ as well as [Ag (tpy)]+ complex cations were reported to be attached electrostatically to the negatively charged outer part of the electric double layer enveloping Ag NPs in hydrosols.12,25,26 Their SERS (and/or SERRS-surface-enhanced resonance Raman scattering) spectra showed no temporal evolution, and their vibrational frequencies and the ground state electronic structure were found to remain unchanged upon this attachment. On the other hand, by a combination of RRS and SERRS excitation profiles evaluation with the results of DFT (density functional theory) calculations, a slight change in localization of one of the electronic transition within the [Fe(tpy)2]2+ complex dication upon its electrostatic bonding


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to Ag NPs (stabilized by adsorbed chlorides) has been revealed.26 In contrast to [Fe(tpy)2]2+, no SERS spectral study of the [Zn(tpy)2]2+ has been reported up to now. In this paper, we demonstrate that the temporal stability of the coordinationally saturated synthetic complexes on chloride-modified Ag NPs surfaces cannot be automatically expected in all cases. In particular, we report an interesting interplay between the bonding of tpy as a ligand in the [Zn(tpy)2]2+ complex dication, and formation of the Ag+s -tpy species that follows after adsorption of [Zn(tpy)2]2+ on the surfaces of Ag NPs modified by adsorbed chlorides. By several time-dependent SERS spectral studies and their treatment by FA, we provide evidence that the [Zn(tpy)2]2+ decomposes on the surfaces of the chloride-modified Ag NPs giving a Zn2+ s -tpy and + Ags -tpy surface species. Furthermore, we report a strategy by which formation of the Ag+s -tpy surface species can be inhibited, and we employ DFT calculations for elucidation of the most probable structure of the Zn2+ s -tpy surface species. In addition to that, we mutually compare the progress and output of the reaction between Ag+s -tpy species and Fe2+ and Zn2+ cations, respectively, and we test the ability of FA to distinguish a contribution of the minority and/or transient spectral component to the SERS spectral set. Finally, we use the approach successfully applied previously to elucidation of the structure of surface complexes on Ag and Au NP surfaces27–34 for elucidation of the actual structure of the Ag+s -tpy surface species. We compared its SERS spectra with the Raman spectra generated by DFT calculations for several proposed model structures , as well as with the Raman spectra of the Ag (I)-tpy complex, for which the actual bonding of tridentate tpy to the Ag (I) central atom(s) and the polymeric character of the complex has been established by single crystal X-ray diffraction.

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EXPERIMENTAL SECTION

2.1

Materials Analytical grade chemicals and distilled deionized or doubly distilled deionized water were used for all sample preparations: FeSO4.5H2O (Lachema), ethanol, methanol (both Merck, UVASOL), NaOH, hydroxylamine hydrochloride, 2,2’:6’,2”-terpyridine, Zn(ClO4)2.6H2O, AgNO3 (all Sigma-Aldrich). 2.2 Preparation procedures 2.2.1 Metal complexes [Zn(tpy)2](ClO4)2.2H2O complex was prepared by mixing a 1x10-2 M solution of tpy in the methanol/water (1:1) mixed solvents with an aqueous solution of Zn(ClO4)2 (1x10-1 M) at the metal/tpy concentration ratio equal to 0.5 by a procedure adopted from Vitvarova et al.18. The structure of [Zn(tpy)2](ClO4)2.2H2O (Figure 1) was verified by single crystal X-ray diffraction of monocrystals (see SI). Crystallographic data for [Zn(tpy)2](ClO4)2.2H2O have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC 1578192. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road,


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Cambridge CG21, EZ, UK (fax: (44) 1223-336-033; e-mail: [email protected]). For details, see SI. [Fe(tpy)2]SO4 was prepared by mixing a 1x10-2 M solution of tpy in the methanol-water mixture (1:1) with a 1x10-1 M aqueous solution of FeSO4. [Ag(tpy)NO3]n complex was prepared by mixing a 1x10-2 M solution of tpy in propanol with a 1x10-1 M solution of AgNO3 in water at the concentration ratio of Ag:tpy 1:1. The resulting compound has formed needle shape crystals which were elastic, light sensitive and showed a strong X-ray absorption. Several different alcoholic solutions were used for the preparation of the Ag complexes, but the observed single crystals own same properties as described above. Moreover, they exhibited complicated twinned modulated structure, which would be probably the subject of a separate crystallographic publication. Here we present a model of crystal structure of [Zn(tpy)2](ClO4)2.2H2O based on the supercell approach (see SI).

Figure 1. Structure of [Zn(tpy)2](ClO4)2.2H2O. The displacement parameters are shown at the 50% probability level. 2.2.2 Ag NP hydrosol Ag NP hydrosol was prepared by reduction of silver nitrate by hydroxylamine hydrochloride according to the preparation procedure described by Leopold and Lendl35, and it is denoted as AgNP throughout this paper. Briefly: 0.3 mL of 1 M aqueous solution of NaOH were added to 90 mL of 1.67×10-3 M aqueous solution of NH2OH.HCl. After that, 10 mL of 1x10-2 M aqueous solution of AgNO3 were added dropwise under vigorous stirring. The resulting sol was dark brown-yellow and opalescent. 2.2.3 SERS active systems For preparation of various SERS active systems, the following amounts of adsorbates and metal salts were added to 1 mL of AgNP hydrosol: 2.5 µL of 1x10-2 M methanol solution of tpy (AgNP–tpy system); 2.5 µL of 5×10-3 M solution of [Zn(tpy)2]ClO4 and [Fe(tpy)2]SO4 (systems labelled as AgNP-[Zn(tpy)2]2+ and AgNP-[Fe(tpy)2]2+), 2.5 µL of 1x10-2 M or 10-1M of aqueous


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solutions of Zn(ClO4)2 or FeSO4, respectively, before addition of tpy or metal complexes to the systems (labelled as AgNP-Zn2+-tpy, AgNP- Zn2+-[Zn(tpy)2]2+, AgNP- Zn2+(10x)-[Zn(tpy)2]2), or after addition of tpy or metal complexes (AgNP-tpy- Zn2+, AgNP-tpy- Fe2+ and AgNP-tpyFe2+(10x) systems). Since no additional chlorides were added to the SERS active systems, the conditions of the Ag(0) adsorption sites generation on Ag NPs surfaces have not been fulfilled, and only the common Ag+ adsorption sites are present on the Ag NPs surfaces stabilized by adsorbed chlorides originating from the Ag NP preparation12. 2.3

Instrumentation Raman and SERS spectra were recorded on a DXR Raman microspectrometer (Thermo Scientific) interfaced to an Olympus microscope (employing an objective 50x for the solid samples and a macro-adapter for liquid samples placed into quartz cells). The 780 nm (diode laser) excitation lines was used. The laser power ranged from 1 mW (for solid samples) to 24 mW for liquid samples. Full range gratings were used (3300 – 40 cm-1, 400 lines/mm). UV/vis and SPE (Surface-plasmon extinction) spectra were monitored on a Specord s600 (Analytik Jena) instrument. 2.4

Analysis of SERS spectral sets The sets of spectra were analyzed using the factor analysis program36 (details in SI).

2.5

Computational details The quantum chemical calculation (Gaussian 09W program package37) of the metal complexes and fragments were performed using the closed-shell restricted density functional theory B3LYP method38,39 with LanL2-DZ effective core potential basis set.40–42 The geometry optimizations were followed by calculations of the vibrational frequency calculations using the same method and basis set. The calculated geometry and frequencies scaled with precomputed vibrational scaling factor43 were compared with the experimental values. The quantum chemical calculation of fragment „P” [Ag4(tpy)5](NO3)4 (Figure 9 in Chapter 3.3.2) was performed using Gaussian 16 program package44. The initial geometry optimization of the selected fragment imposed no geometry restraints and led to a complete rearrangement of the structural motif that was triggered by the significant increase of the intermetallic distances. To successfully retain the helical structural motif in the absence of external forces, the geometry optimization was repeated by freezing the three intermetallic distances and two angles between the metals at their experimental values, while leaving simultaneously all other structural parameters free to vary. The theoretical Raman intensities of the computed normal modes were calculated (RAINT program45) for the 780 nm excitation wavelength, taking the Raman scattering activities from the Gaussian output.


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RESULTS AND DISCUSSION

Metalation markers in Raman and SERS spectra of [Zn(tpy)2]2+, [Ag(tpy)]+, [Fe(tpy)2]2+ complexes and indications of surface reactivity of [Zn(tpy)2]2+ A comparison of Raman spectra of solid microcrystalline samples of [Zn(tpy)2](ClO4)2.2H2O [Fe(tpy)2]SO4 and [Ag(tpy)NO3]n complexes excited at 780 nm (Figure S5B) shows that their most pronounced mutual differences are observed in the 950-1050 cm-1 region in which the breathing vibrations of the coordinated tpy ligand are observed. The most intense bands in this region are located at 1029 cm-1 for the [Zn(tpy)2](ClO4)2.2H2O, at 1047 cm-1 for [Fe(tpy)2]SO4 and at 1008 cm-1 for [Ag(tpy)NO3]n, and they are assigned as tpy metallation marker bands specific for each of the central metal ions. While Raman spectra of the [Fe(tpy)2]SO4 and the [Ag(tpy)NO3]n complexes have been reported previously12,26, the single crystal X-ray structure (Figure 1 and SI) and the Raman spectrum (Figure 2, spectrum a) of [Zn(tpy)2](ClO4)2.2H2O are newly reported in this paper. Both the [Zn(tpy)2]2+ (Figure 1) and [Fe(tpy)2]2+ complex cations possess the distorted octahedral structures23,24,46 with the central pyridine rings more strongly bonded to the central metal ions than the distal ones, however, the Zn-N bonds are (Table S2), in general, longer than the Fe-N ones. This result, in accord with the wavenumbers of the coordinated tpy breathing modes (which are higher for [Fe(tpy)2]2+ than for [Zn(tpy)2]2+, as shown above) indicates that tpy is more strongly coordinated to the Fe2+ central metal ion than to the Zn2+ one. 3.1

Figure 2. Raman spectra of solid [Zn(tpy)2](ClO4)2.2H2O (a), SERS spectrum of AgNP-[Zn(tpy)2]2+ system (b), SERS spectrum of AgNP-tpy system (c). (*ClO4- anion) The SERS spectrum of the AgNP-tpy system obtained at 780 nm excitation (Figure 2, spectrum c and Figure S5A) contains the intense Ag+-tpy metallation marker band at 1008 cm-1. It indicates the presence of the Ag+-tpy surface species structurally similar to the synthetically prepared [Ag(tpy)NO3]n (Figure S5B). The SERS spectra of the [Zn(tpy)2]2+ (Figure 2, spectrum b and Figure S5A) and [Fe(tpy)2]2+ (Figure S5A and ref.26) complex dications revealed a


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dramatically different faith of each of the two complexes upon their adsorption on the surfaces of chloride-stabilized Ag NPs. The SERS spectrum of [Fe(tpy)2]2+ obtained from the AgNP[Fe(tpy)2]2+ system (Figure S5A and ref.26) is largely similar to the solid state Raman spectrum of this complex dication (Figure S5B and ref.26). In both cases, the 1047 cm-1 metallation marker band is the most intense band in the breathing modes spectral range. By contrast, the SERS spectrum of the AgNP-[Zn(tpy)2]2+ system (Figures 2, spectrum b and S5B) differs from the Raman spectrum of this cation in the [Zn(tpy)2](ClO4)2.2H2O complex (Figures 2, spectrum a and S5B), and markedly resembles that of AgNP-tpy (Figure 2-c and S5-A). Both the Zn2+-tpy (1029 cm-1) and Ag+-tpy (1008 cm-1) metallation markers are observed in the spectrum of the AgNP-[Zn(tpy)2]2+ system (Figure 2, spectrum a). The Ag+-tpy metallation marker being more intense than the Zn2+-tpy one. These observations strongly indicate a partial decomposition of the [Zn(tpy)2]2+ complex dication and the presence of some Ag+-tpy species on the surface of chloride-stabilized Ag NPs. In the subsequent sub-Chapter 3.2, we present the results of several time-dependent SERS spectral studies and of their evaluations by FA targeted on providing an unequivocal evidence for the progress of this reaction and on finding possible pathways to its inhibition.

Figure 3. A - Time evolution of SPE spectra of AgNP-[Zn(tpy)2]2+ system, B - SPE spectrum measured 1000 s after system preparation, C - time evolution of SPE at 780 nm. Probing the surface chemistry in systems with AgNPs and [Zn(tpy)2]2+ by time-evolution of SERS spectra and their factor analysis 3.2.1 AgNP-[Zn(tpy)2]2+ system Selection of a suitable excitation wavelength was a necessary prerequisite to investigation of the SERS spectral time-evolution of the AgNP-[Zn(tpy)2]2+ system. Towards this goal, the time evolution of SPE spectrum of AgNP-[Zn(tpy)2]2+ system has been followed (Figure 3). The rate of aggregation, which manifests itself by the appearance of the band at ca 700 nm, was the highest during the first 30 s. On the basis of the SPE signal time evolution at the wavelengths corresponding to the available Raman excitations (532, 633 and 780 nm), the 780 nm excitation 3.2


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wavelength was selected for the measurements of the time evolution of SERS spectra. At this wavelength (unlike at the 532 and 633 nm wavelengths) the SPE reached its nearly steady value after ca 270 s, and started to decrease at ca 1500 s (Figure S6). Nevertheless, a measurement of the time evolution of the SERS active systems was stopped already after ca 1000 s. The onset of the aggregates sedimentation in the system was already observable by a naked eye, as the upper part of the system started to discolorate and a sediment started to formed at the bottom of the cuvette. Finally, it should also be noted that another advantage of selecting the 780 nm excitation for the time-dependent SERS spectral measurements is the absence of any molecular resonance contribution to the Raman and/or SERS spectra of both the surface and the synthetic complexes measured at this excitation (as follows from the previously published data12,18,26).

Figure 4. Time-evolution of SERS spectra of: A – AgNP-[Zn(tpy)2]2+, B – AgNP-tpy-Zn2+, C – AgNP[Zn(tpy)2]2+-Zn2+(10x), D – AgNP-Zn2+-[Zn(tpy)2]2+, E – AgNP-Zn2+(10x)-[Zn(tpy)2]2+ systems. The blue lines – the first, the black lines – the last measured spectra of the time evolutions. The red


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line in spectrum B belongs to the spectrum of AgNP-tpy, i.e. measured before the addition of Zn2+ ions. The time-evolution of the SERS signal of Ag NP-[Zn(tpy)2]2+ system (Figure 4A) has revealed several spectral changes. The most striking ones were observed in the region of the breathing vibrations. The intensity of the Ag+-tpy metalation marker (1008 cm-1) together with that of the mode at 1043 cm-1 increased, whereas the Zn2+-tpy metalation marker (1029 cm-1) showed an intensity decrease. Furthermore, spectral shifts were observed in the region of ν(CC) and ν(CN) stretching vibrations (1420-1520 cm-1, inset in Figure 4A). The obtained spectral set was treated by the FA. The results of FA (Figure S7), namely the values of singular numbers (Wj) and residual errors indicate the presence of two spectral components. The first subspectrum S1 is a weighted average of the experimental spectra and reflects the overall intensity changes, while the second subspectrum S2 shows the major spectral changes within the spectral set: an increase of the Ag+-tpy and a decrease of the Zn2+-tpy metalation spectral markers and a decrease of the band intensities in the stretching vibrations region (1200-1600 cm-1) together with some spectral shifts (mainly 1332 to 1323 cm-1 and others depicted in Figure 4A).


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Figure 5. SERS spectra of pure spectral components obtained by factor analysis of the particular set(s) of time-evolving SERS spectra.

+ Figure 6. Time-evolution of the relative contributions of Zn2+ s -tpy and Ags -tpy pure spectral components to the overall SERS signal obtained from each of the particular SERS active systems denoted by the headings in graphs A-E.


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All the observed spectral changes indicate that [Zn(tpy)2]2+ complex undergoes a decomposition upon adsorption on the Ag NP surface. The released tpy interacts with the Ag+ adsorption sites on Ag NP surface and a surface species denoted as Ag+s -tpy is formed. The pure spectral components P1 and P2 were constructed by linear combination of the first two subspectra of a data set created from all time evolution spectral sets described in this paper and containing Ag NPs and [Zn(tpy)2]2+ complex (vide infra). The spectrum of pure component P1 attributed to the Ag+s -tpy surface species (Figure 5) is nearly identical with the Raman spectrum of [Ag(tpy)]NO3 complex (Figure S5B) and identical with SERS spectrum of the AgNP– tpy system (Figure 2, spectrum c). The spectrum of the pure spectral component P2 (Figure 5) resembles the Raman spectrum of [Zn(tpy)2](ClO4)2.2H2O (Figure 2, spectrum a) and it is thus attributed to a Zn2+ s -tpy surface species. Decomposition of the measured set of the SERS spectra of the Ag NP-[Zn(tpy)2]2+ system into pure components provided the time evolution of the relative contributions of each of the pure spectral forms to the overall SERS spectrum. The spectral component P1 (Ag+s -tpy surface species) becomes the dominant component (77%, Figure 6A) of the SERS spectrum already ca 20 s after the system preparation (i.e. in the first measured spectrum in Figure 4A - blue line), and reaches 89 % participation after 1000 s (ca 16 min, Figure 6A) of the system evolution (Figure 4A, black line). The decomposition of the [Zn(tpy)2]2+ complex dication thus starts almost immediately after its addition to Ag NPs hydrosol. Furthermore, our intention was to establish whether the decomposition of the [Zn(tpy)2]2+ complex dication proceeds also upon addition of Ag+ ions to the solution. Owing to the low concentration of the complex in the studied solution (vide infra), Raman spectroscopy could not be used for monitoring of such process, hence UV/vis spectroscopy (used previously for following formation of this complex dication in solution18) was employed. The UV/vis spectral monitoring of the titration of a 5×10-5 M solution of [Zn(tpy)2]2+ in methanol/water (1:1) solution by an equimolar aqueous solution of AgNO3 has not shown any substantial changes in the [Zn(tpy)2]2+ UV/vis spectra. It indicates that the [Zn(tpy)2]2+ complex dication is stable in the presence of Ag+ ions under these conditions (Figure S8). Hence its decomposition on the chloride-stabilized Ag NP surfaces should be a surface reaction indeed, conditioned by a concerted action of Ag+ adsorption sites and Cl- ions (vide infra) on Ag NP surfaces. This surface reaction thus prevents obtaining meaningful SERS spectra of the [Zn(tpy)2]2+ complex, since the major contribution to the obtained SERS signal originates from the Ag+s -tpy surface species. 3.2.2 AgNP-tpy-Zn2+ system In the next step, we tried to generate the Zn2+ s -tpy surface species directly on the surface of the Ag NPs modified by adsorbed tpy. Therefore, the equimolar amount of Zn(ClO4)2 was added to the AgNP-tpy system, forming so the system denoted as AgNP-tpy-Zn2+. The time evolution of the SERS spectrum was followed for 1000 s (i.e. ca 16 min, Figure 4B). Immediately after


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addition of Zn2+ ions, the Zn2+-tpy metalation marker at 1029 cm-1 appeared and its intensity increased in time course. Minor spectral changes were observed also in the stretching vibrations region (1400 – 1600 cm-1). FA of the spectral set (Figure S9) revealed the meaningful contributions of the first two subspectra to the overall spectral set. Analogously to the case of the AgNP-[Zn(tpy)2]2+ system (sub-Chapter 3.2.1), the spectra of two pure spectral components P1 and P2 were constructed, and they were attributed to the Ag+s -tpy and the Zn2+ s -tpy surface species, respectively (Figure 5) . The time evolution of the relative contribution of each spectral form to the overall SERS spectrum (Figure 6B) showed that a small fraction (ca 13 %) of the pure spectral component P2 attributed to the Zn2+ s -tpy surface species contributed to the overall SERS spectrum of the system already in the first spectrum after the addition of the Zn2+ ions (blue spectrum, Figure 4B). Nevertheless, the subsequent SERS spectral evolution showed that the spectral contribution of the Zn2+ s -tpy species reached only ca 20 % after 16 min, while the majority of the SERS signal still originated from the Ag+s -tpy surface species. Equally low yield of 2+ Zn2+ s -tpy surface species (ca 22 %) was observed even if the tenfold molar excess of the Zn ions with respect to tpy was used.

3.2.3 AgNP-[Zn(tpy)2]2+-Zn2+(10x) In the next experiment we tried to prevent the decomposition of the [Zn(tpy)2]2+complex during SERS measurement by subsequent addition (ca 30 s after system preparation) of the tenfold excess of Zn2+ ions (as Zn(ClO4)2) with respect to [Zn(tpy)2]2+ to the AgNP-[Zn(tpy)2]2+ system. Our aim was to probe, whether the strategy previously employed for inhibition of metal ions exchange in solution47 can succeed also in the case of a surface reaction. The SERS spectral time evolution of the resulting AgNP-[Zn(tpy)2]2+-Zn2+(10x) system (Figure 4C) shows that the addition of Zn2+ ions stopped the decomposition of the zinc complex, but did not induce any distinct progress in the intensity of Zn-metalation marker. The overall changes in SERS spectra were almost negligible during the whole measurement. The FA of the AgNP-[Zn(tpy)2]2+-Zn2+(10x) spectral set (Figure S10) revealed, as in the previous cases (sub-Chapters 3.2.1 and 3.2.2), a meaningful contribution of the first two subspectra to the overall spectral set. The time evolution of the relative contributions of each of the two pure spectral forms to the overall SERS spectrum (Figure 6C) showed that the initial contribution of the of P1 spectral component attributed to the Zn2+ s -tpy surface species to the overall SERS signal was 22 % , and it slowly increased to 29 % after 16 min. Nevertheless, the P2 spectral component assigned to the Ag+s -tpy surface species with its 71 % contribution still remained the major component contributing to the SERS spectrum. Therefore, this strategy of the supplementary addition of the Zn2+ ions to the AgNP-[Zn(tpy)2]2+ system only slightly suppressed, but did not efficiently inhibit formation of the Ag+s -tpy species on the surfaces of chloride-stabilized Ag NPs.


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3.2.4 AgNP-Zn2+-[Zn(tpy)2]2+ and AgNP-Zn2+(10x)-[Zn(tpy)2]2+ systems Since the subsequent addition of the excess of Zn2+ ions to the AgNP-[Zn(tpy)2]2+ system did not lead to fulfillment of our aim, the strategy was changed. We tried to prevent formation of the Ag+s -tpy species by addition of Zn2+ ions to the Ag NPs hydrosol prior to the administration of the [Zn(tpy)2]2+ complex. Already the addition of an equimolar amount of Zn(ClO4)2 (in respect to the [Zn(tpy)2]2+, the system denoted as AgNP- Zn2+-[Zn(tpy)2]2+) caused that the most intense breathing vibration band was the Zn2+-metalation marker. Moreover, the overall spectral pattern (Figure 4D) resembled Raman spectrum of the [Zn(tpy)2](ClO4)2 complex (Figure 2, spectrum c). FA of the AgNP-Zn2+-[Zn(tpy)2]2+ SERS spectral set revealed a meaningful contribution of the first two subspectra to the overall spectral set (Figure S11). Decomposition of the SERS spectral set into pure components P1 and P2 (Figure 6D) proved that the SERS signal at the beginning of the time evolution originated in 67% of Zn2+ s -tpy surface species (spectral component P2, Figure 5) and raised slowly during 16 min to 71% in the last spectrum (Figure 6D). The administration of tenfold excess of Zn2+ ions prior to the [Zn(tpy)2]2+ complex addition to the Ag NPs (AgNP-Zn2+(10x)-[Zn(tpy)2]2+system, Figure 4E) suppressed formation of the Ag+s tpy species on the surfaces of chloride-stabilized Ag NPs to only 10%, but not completely. Nevertheless, the SERS spectral pattern was mostly consistent with the spectrum of Zn2+ s -tpy surface species. The only difference was a slightly higher relative intensity of the 1008 cm-1 band. The FA of the spectral set (Figure S12) showed only one meaningful subspectrum. The reason is that the SERS spectrum shows virtually no temporal evolution, and thus represents a set of nearly constant spectra representing a (very rapidly formed) steady state. This is demonstrated by the time-dependence of the relative contributions of each of the spectral components P1 and P2 to the overall SERS signal (Figure 6E). This plot also shows that the P2 spectral component attributed to Zn2+ s -tpy surface species (Figure 5) reached 86 %, i.e. a largely dominant contribution to the overall SERS signal. In summary, it has been demonstrated that administration of the Zn2+ cations into Ag NP hydrosol prior to [Zn(tpy)2]2+ complex efficiently prevented formation of the (undesired) Ag+s tpy surface species and enabled to obtain SERS spectra of the Zn2+ s -tpy surface species. The zinc ions have to be administrated prior to the studied complex, i.e., they should occupy the electric double layer of the Ag NPs to prevent formation of Ag+s -tpy. The once formed Ag+s -tpy surface complex is so stable that even the subsequent administration of tenfold excess of Zn2+ ions did not shift the equilibrium between the Ag+s -tpy and Zn2+ s -tpy surface species in favor of the latter one.


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3.3 DFT calculations 3.3.1 Elucidation of the Zn2+ s -tpy surface species structure The performed FAs have shown that all the time-evolving SERS spectra in all the above data sets can be described as a sum of contributions of only two pure spectral components, P1 + attributed to the Zn2+ s -tpy surface species and P2 assigned to the Ags -tpy surface species (Figures 5 and 6A-E). In addition, the time-dependent SERS spectral measurements of the AgNP[Zn(tpy)2]2+ system (Figure 6A) have shown that the Zn2+ s -tpy surface species cannot be 2+ identical with the parent [Zn(tpy)2] complex dication, since the spectrally clearly distinguished Ag+s -tpy surface species could only be formed from the tpy ligands released by decomposition of the [Zn(tpy)2]2+ complex on the surface of chloride-modified Ag NPs.

Figure 7. Comparison of SERS spectral pure component of Zn2+ s -tpy surface species (a) and calculated Raman spectra of various Zn – tpy complexes and fragments: [Zn(tpy)Cl2] (b), [Zn(tpy)Cl]+ (c), [Zn(tpy)2]2+ (d), [Zn(tpy)]2+ (e).


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Experimental data showed a fast ligand exchange between [Zn(tpy)2]2+ species and surface Ag+ ions, which gives Ag+s -tpy surface species and species of the [Zn(tpy)]2+ type. Since the whole process is located on the Ag NPs surface or in its close vicinity (as witnessed by the possibility to follow the process by SERS), the Cl- anions present there due to reduction of AgNO3 by NH2OH.HCl during the preparation of Ag NP35 are the most probable reaction partners for [Zn(tpy)]2+ cation. To elucidate the most probable structure of the Zn2+ s -tpy surface species, we employed the DFT calculations of theoretical Raman spectra of [Zn(tpy)2]2+, [Zn(tpy)]2+, [Zn(tpy)Cl]+ and [Zn(tpy)]Cl2 complex species. The comparison of the theoretical Raman spectra to the SERS spectrum of Zn2+ s -tpy surface species obtained by FA of studied systems (vide supra) is shown in Figure 7. The best match was obtained for the structures involving one or two Cl- ligands coordinated to the Zn2+ central ion (Figure 7, spectrum b and c). Both the spectra are very similar and their spectral pattern resembled the SERS spectrum of Zn2+ s -tpy surface species in both the breathing and the CC and CN stretching vibration regions. Since the complex [Zn(tpy)Cl2] has been already described in literature48–50, we suppose that the most probable structure of Zn2+ s -tpy surface species is represented by a complex in which the coordination sphere of the Zn dication contains one tpy ligand and one or two chloride anions. Furthermore, since the Zn2+ s -tpy surface species is the only surface species containing the 2+ Zn dication and tpy detected by FA, it appears that the parent [Zn(tpy)2]2+ species undergo a very rapid decomposition within about half a minute after the system preparation. This observation proves the high lability of [Zn(tpy)2]2+ species, which is in accordance with the fast constitutional dynamics observed for metallo-supramolecular polymers derived from α,ωbis(tpy)oligothiophenes and Zn2+ ions.51–53 3.3.2 Elucidation of Ag +s -tpy surface species structure Elucidation of the structure of the Ag+s -tpy surface complex was carried out in three steps. First, the experimental spectra, namely the SERS spectrum of Ag+s -tpy surface species obtained from the FA (Figure 5), the SERS spectrum of of AgNP-tpy system (Figure S5-A) and the the Raman spectrum of the synthetically prepared [Ag(tpy)NO3]n complex (Figure S3) were mutually compared (Figure 8, spectra a, b). The SERS spectrum of Ag+s -tpy surface species obtained from the FA (Figure 5) is in full agreement with the SERS spectrum of AgNP-tpy system (Figure S5-A), which indicates that, under the applied conditions, only one form of Ag-tpy surface complex was present. Moreover, the SERS spectrum of Ag+s -tpy surface species is as to the band positions identical with the Raman spectrum of the comparative [Ag(tpy)NO3]n complex (Figure 8, spectra a, b) and differs only in the relative intensities of vibrational bands. Furthermore, the spectrum-structure relationship has been established for the [Ag(tpy)NO3]n complex by a partial solution of its X-ray structure (the details and the reasons preventing a full solution in are provided in SI). The X-ray structure has shown that the unit cell


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contains 4 helices (Figure S4) in which the tpy acts as a bridging ligand. In particular, the tpy is bidentatelly coordinated to one Ag atom and its 3rd pyridine ring is monodentatelly coordinated to the adjacent Ag atom. The pseudotetrahedral coordination sphere of silver has thus been achieved. It should be noted that although the typical coordination sphere of Ag+ ions is tetrahedral, other coordinations of tpy in Ag – tpy crystals have been published: Constable et al.54 and Hannon et al.55 prepared square-planar silver complexes and Bin Silong et al.56 prepared a complex with mixed coordination in a helical structure.

Figure 8. SERS Ag+s -tpy surface species (a), Raman spectrum of polymeric [Ag(tpy)NO3]n complex (b), Raman spectrum obtained by DFT calculations of Ag-tpy fragment P (c) (see also Figure 9). Bands marked by asterisk (*) are mainly attributed to the NO3- vibration modes.


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Figure 9. Ag – tpy fragments used for DFT calculations of Raman spectra. H atoms were omitted for the sake of clarity. In the second step, we focused on elucidation of the mode of coordination of tpy to Ag+ cations in the Ag+s -tpy surface complex. For this purpose, we employed the ab initio DFT calculations of several Ag+-tpy fragments (Figure 9 – structures I - IX). The selections of more probable fragments were based on the metal structures involved in the Cambridge Structural Databases (CSD).57 Furthermore, for a meaningful comparison of the SERS spectrum of Ag+s -tpy (as the pure spectral component) with the Raman spectra of model fragments resulting from the DFT calculations, the spectral marker bands which strongly reflect the different modes of tpy coordination to Ag+ have been selected: (i) the breathing modes with bands in the 950-1050 cm-1 region (Figure S13-A and their colored details in Figure S13-B ) and (ii) the CC and CN in plane stretching or deformation modes in the 1500-1600 cm-1 region (highlighted by a grey rectangle in Figure S13-A). By their comparison, the following indices for description of Ag+s -tpy structure have been obtained. First, all three pyridine rings of tpy are coordinated to Ag+ since the breathing vibration of the Ag+s -tpy at 1008 cm-1 is a single band with only a small shoulder at ca 995 cm-1 (Figure 5), which mostly corresponds to the structures V and IV (Figure S13). Nevertheless, the structure


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IV is highly improbable, since no such (i.e. pseudo-octahedral) coordination was described for the Ag+ central ion.57 The structure III has two very closely laying (distance ca 15 cm-1) breathing bands – an out-of-phase band at 990 cm-1 and a in-phase band at 1005 cm-1 that can be joined together forming one band only if the half-widths of bands are more than ca 10 cm-1, which is approximately twice more than observed in the real spectra of Ag+s -tpy surface species. Secondly, in the spectral range of the CC and CN vibrations (1500-1600 cm-1) (Figure S13-A) the best match has been obtained for the structures II, and VII, where a doublet in the 1550-1600 cm-1 region, in contrast to the triplet (structures V,III and I) or a main peak with a small shoulder at lower wavelength (IV, VI, VIII and IX), has been observed. In both the structures II and VII, tpy is coordinated to one Ag+ ion with only two or one pyridyl unit. These indices, i.e. coordination of all N atoms of the tpy ligand to at least two Ag+ atoms is consistent with the X-ray structure determined for the synthetically prepared [Ag(tpy)NO3]n . Therefore, in the third step, we performed the DFT calculation of a fragment formed by four Ag+ ions and five tpy units and NO3- anions, based on the bridging coordination of tpy units, and for the sake of electroneutrality and convergence of the calculation, accompanied by NO3anions (Figure 9, structure P). The model of this polymeric structure (based on the structural motif of [Ag(tpy)NO3]n crystal, Figure S3) involves both structural characteristics determined above: all pyridine rings of tpy are coordinated, and tpy acts as a bridging ligand, i.e. it is coordinated to one Ag+ ion with only two or one pyridyl unit. The calculated Raman spectrum of this polymeric fragment (Figure 8, spectrum c) provides a good agreement with the observed SERS spectrum of Ag+s -tpy surface species in both the breathing and CC and CN stretching vibration regions. Therefore, we propose that the tpy molecule in Ag+s -tpy surface species acts as a bridging ligand between two adjacent Ag+ surface ad atoms on the surface of Ag NPs forming thus a polymeric surface species.

Time-evolution of SERS spectra and FA of the AgNP-tpy-Fe2+ and AgNP-tpy-Fe2+(10x) systems To test the ability of FA to distinguish a minority and/or transient component within the SERS spectral evolution, we have designed and preformed the following experiment. To design the experiment, we used the previously acquired information about the surface stability of the [Fe(tpy)2]2+ complex dication. This stability manifests itself by the lack of any time-evolution of the SERS signal of this complex26, as well as by the very close match between its Raman and SERS spectrum (Figure S5 and ref26). In the experiment itself, we followed the time evolution of the SERS signal during generation of the [Fe(tpy)2]2+ complex on the surface of chloridestabilized Ag NPs modified by adsorbed tpy by subsequent addition of Fe2+ cations, and we treated the SERS spectral set by FA. 3.4


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SERS spectral time-evolution of the AgNP-tpy-Fe2+ (equimolar amount of Fe2+ in respect to tpy) and of the AgNP-tpy-Fe2+(10x) (tenfold excess of Fe2+) systems is shown in Figures 10A and B, respectively. The SERS spectral features of Fe2+ s -tpy surface species appeared already in the first spectrum measured after the addition of Fe2+ ions to the AgNP-tpy system. The major changes observed in the spectra are appearance of the Fe2+-tpy metallation marker at 1047 cm-1, and shifts of the bands of ring stretching vibrations: (i) upshift of the 1323 cm-1 band to 1332 cm-1 and of the 1594 cm-1 band to 1598 cm-1, (ii) downshift of the 1290 cm-1 band to 1286 cm-1, and (iii) intensity increase of the band at 1471 cm-1.

Figure 10. SERS spectra of AgNP-tpy systems after addition of: (A) equimolar amount of Fe2+ ions and (B) tenfold molar excess of Fe2+ ions (with respect to tpy). Red - before addition of Fe2+, violet – the 1st spectrum after addition of Fe2+ ions, black spectrum – 800 s after addition of Fe2+ ions . The FAs of both temporal SERS spectral sets showed that not only the first two, but also the 3 subspectrum (S3) meaningfully contributes to the overall SERS spectral set (Figures S14 and S15). The most pronounced changes of Vi3 coefficients were observed for the first 90 s (the first 5 spectra) in AgNP–tpy–Fe2+ system (Figure S14) and for the 54 s (the first 3 spectra) of AgNPtpy-Fe2+(10x) system (Figure S15). The signal-to-noise ratio in 3rd subspectrum is markedly better for the system with the equimolar amounts of the Fe2+ ions and tpy than in the system with the tenfold excess of Fe2+. Linear combination of the subspectra S1 and S2 obtained by FA of Ag NP-tpy- Fe2+ as well as of Ag NP-tpy- Fe2+(10x) systems (Figures S14, S15) provided identical spectra of pure components (P1 and P3, Figure 5) in both cases. These spectra were attributed to the Ag +s -tpy and Fe2+ s -tpy surface species. The former pure component SERS spectrum closely matches that of the AgNP-tpy system (Figure S5A), and the latter one is in a very good agreement with both SERS (Figure S5A) and Raman spectra (Figure S5B) of rd


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[Fe(tpy)2]2+. The spectral information involved in the 3rd and all subsequent subspectra represented only ca 1 - 2 % of the total signal (the difference between the original spectrum and spectrum reconstructed using the 1st and the 2nd subspectrum).

+ Figure 11. Time-evolution of the relative contributions of Fe2+ s -tpy and Ags -tpy pure spectral components to the overall SERS signal obtained from each of the particular SERS active systems denoted by the headings in graphs A and B.

Moreover, the FAs of the reduced SERS spectral sets obtained for aging times from ca 250 s to 800 s (the 15th to the 50th spectrum) have shown that only the first two subspectra S1 and S2 are necessary for the reconstruction of the spectral set. These results indicate that even if we were not successful in obtaining the SERS spectrum of the third pure component (owing to its very low contribution to the overall SERS spectral set), we were able to establish its presence and tentatively attribute it to an intermediate of the [Fe(tpy)2]2+ generation, most probably to a surface complex of Fe2+ and one tpy unit. Furthermore, the each of the two SERS spectral sets was described in terms of the relative contributions of the two pure components (attributed to the Ag +s -tpy and of the Fe2+ s -tpy surface species, respectively in Figure 5) to overall SERS signal, as shown in Figure 11. In particular, the addition of an equimolar amount of FeSO4 to the Ag NP–tpy system (Ag NP-tpyFe2+) caused formation of the Fe2+ s -tpy surface species which manifested itself by the increase of its relative contribution to the overall SERS signal from 4 % to 17 % during the overall time evolution (Figure 11A). The 10-fold higher addition of FeSO4 to the Ag NP-tpy system (AgNP-tpy-


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Fe2+(10x) system) caused a rapid increase of the Fe2+ s -tpy SERS signal from 27 % in the first measured spectrum to 47 % after 5 min and remained unchanged for the next 8 min (Figure 11B) when the SERS measurement was finished. In retrospect, the absence of any 3rd subspectrum in the FA of spectral sets involving the Zn2+ s -tpy surface species (attributed to the [Zn(tpy)Cl2] complex in the close vicinity of Ag NP surface, sub-Chapter 3.3.2) indicates that there is no intermediate Zn2+- and tpy- containing component involved in any of the measured SERS spectral sets treated by FA in Chapter 3.2. This result thus confirms that a very rapid (within less than a half-minute) decomposition of the [Zn(tpy)2]2+ into the [Zn(tpy)Cl2] complex takes place on the surfaces of chloride-modified Ag NPs in hydrosols. Interestingly, one can also note some parallel between the observed difference in the stability of the [Zn(tpy)2]2+ and [Fe(tpy)2]2+ ions on chloride-modified Ag NP surfaces and the results of recent studies on metallo-supramolecular polymers derived from α,ω-bis(tpy)oligomers and metal ions that showed a fast constitutional dynamics of the polymers with Zn2+ ions but the very slow one for those with Fe2+ ions.51–53

4

CONCLUSIONS

Evidence of a surface-induced partial decomposition of [Zn(tpy)2]2+ complex ions on the surfaces of chloride-stabilized Ag NPs upon formation of Ag+s -tpy and Zn2+ s -tpy surface species has been obtained by measurements of the temporal evolution of SERS spectra of the AgNP[Zn(tpy)2]2+ system. Factor analysis (FA) of the set of the time-dependent spectra provided SERS spectra of two pure spectral components, namely Ag+s -tpy and Zn2+ s -tpy, as well as the time evolution of their relative contributions to the overall spectral set. Addition of Zn2+ cations to an Ag NPs hydrosol prior to addition of [Zn(tpy)2]2+ has been shown to substantially inhibit the Ag+s -tpy surface species formation. In particular, the final SERS spectrum dominated by the 2+ contribution of Zn2+ s -tpy species (86 %) has been obtained at the tenfold excess of Zn ions with respect to the complex ions. By contrast, the addition of the tenfold excess of Zn2+ ions after the addition of [Zn(tpy)2]2+ complex cations to the hydrosol of Ag NPs reduced the formation of Ag+s -tpy species only marginally. Furthermore, in contrast to the systems with the [Zn(tpy)2]2+ complex cations, no time-evolution of the SERS spectrum was observed for the [Fe(tpy)2]2+ complex cations adsorbed on the Cl--stabilized Ag-NPs, which proves stability of [Fe(tpy)2]2+ under these conditions. The most probable structure of the Zn2+ s -tpy surface species has been established as [Zn(tpy)Cl2] on the basis of a comparison of the pure component Zn2+ s -tpy SERS spectra with the Raman spectra of several model structures obtained by DFT calculations. The DFT calculations were also employed for elucidation of the structure of Ag+s -tpy surface species, and their results were combined with the structural data on the synthetically prepared [Ag+(tpy)]n complex obtained by the single crystal X-ray diffraction. These data indicate a bridging


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coordination of tpy between two Ag+ ion (and/or an adsorption site), in which a tpy ligand is bound by two nitrogen atoms to one Ag+ ion (and/or an adsorption site), and by one nitrogen atom to the near neighboring Ag+ ion and/or site. As a counterpart to the investigation of the reactivity and/or stability of the [Zn(tpy)2]2+ and [Fe(tpy)2]2+ complexes on surfaces of Cl--stabilized Ag NPs surfaces, we also studied the stability of the Ag+s -tpy surface species in the presence of Zn2+ and Fe2+ ions. The FA evaluation of time-evolutions of SERS spectra of AgNP-tpy system upon additions of the respective ions has revealed, in both cases, a partial decomposition of the Ag+s -tpy species accompanied with 2+ the formation of the corresponding Zn2+ s -tpy and Fes -tpy surface species, respectively, on the surfaces of the chloride-stabilized Ag NPs. The former surface species has been identified as the [Zn(tpy)Cl2] complex. Its structure indicates that the presence of the residual chlorides (originating from the hydrosol synthesis) on Ag NP surfaces has played an important role in its formation. The SERS spectral sets obtained for the AgNP-tpy-Zn2+ systems were found to be constituted by two pure spectral components ( Ag+s -tpy and Zn2+ s -tpy), the relative contributions of which were almost independent on the concentration of Zn2+ ions. On the other hand, the FA of the sets of time-evolving SERS spectra of the AgNP-tpy-Fe2+ systems revealed a minor contribution of a third pure spectral component that has been tentatively attributed to a mono-tpy Fe2+ species. This transient spectral component contributed to the SERS spectral set only for the first few minutes, and the rest of the SERS spectral evolution set could be fully reconstructed by the relative contributions of the pure components spectra of the Ag+s -tpy and Fe2+ s -tpy surface species. On the basis of the match between the pure

component SERS spectrum of the Fe2+ s -tpy surface species and the SERS spectrum of the 2+ [Fe(tpy)2] (identical with the Raman spectrum of [Fe(tpy)2]2+ and temporally stable on the surface of Cl--stabilized Ag NPs), the Fe2+ s -tpy surface species has been attributed to the 2+ [Fe(tpy)2] complex. In addition to that, the formation of this complex in the AgNP-tpy-Fe2+ system was found to be positively affected by an increase of the amount of the Fe2+ cations added into the system. In more general terms, we demonstrate that a SERS spectral measurement of a coordinationally saturated complex cation on the surface of the chloride-stabilized Ag NPs (resulting from the very common Ag NP hydrosol preparation) can be seriously hampered by a surface reaction, such as the ligand-exchange reaction between chlorides stabilizing the Ag+ adsorption sites on Ag NP surfaces and one tpy ligand of the [Zn(tpy)2]2+ complex, which resulted, in this particular case, into formation of the [Zn(tpy)Cl2] and the [Ag+(tpy)]n surface species. Finally, by elucidation of the most probable mechanism of this surface reaction and by finding a possible pathway to its control, we demonstrate the unique potential of FA treatments of time-dependent SERS spectral studies for revealing the mechanisms of surface reactions undergone by molecules on plasmonic NP surfaces. Furthermore, a comparison of the pure component spectra of the surface species resulting from FA with the experimental spectra


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of their synthetic analogues (with the established structure-spectrum relationship) and with the spectra of model structures obtained by DFT calculations emerge as a powerful tool for identification of the products of the abovementioned surface reactions.

ACKNOWLEDGEMENT This work was supported by the Czech Science Foundation (17-05007S). The authors thank to Dr. Miroslav Šlouf for writing programs for decomposition of spectral sets into pure components and to Dr. Robert Gyepes for help with DFT calculations. Crystallographic data of [Zn(tpy)2](ClO4)2.2H2O and [Ag(tpy)NO3]n, factor analysis description, SERS and Raman spectra of Zn(II), Fe(II) and Ag(I)-tpy complexes, SPE spectra of AgNP-[Zn(tpy)2]2+ system, UV/vis spectra of water-methanol solution of [Zn(tpy)2]2+ and [Ag(tpy)]+ complexes, results of FA of all studied systems, calculated Raman spectra of Zntpy and Ag-tpy complexes and fragments. Supporting Information Available:

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Probing Formation, Structure and Reactivity of Zn(II ... - ACS Publications

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