SERS, Raman, IR and DFT investigation of 1-(2-pyridylazo ...

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SERS, Raman, IR and DFT investigation of 1-(2-pyridylazo)-2-naphthol and its metal complexes

L. Szabó1*, K. Herman1, N. Leopold1, A. Fălămaş1, N. Mircescu1, C. Buzumurgă2, V. Chiş1

1Faculty of Physics, Babeş-Bolyai University, Kogalniceanu 1, 400084 Cluj-Napoca, Romania 2 “Nicolae Stăncioiu" Heart Institute, Moţilor 19-21, 400001 Cluj-Napoca, Romania

*laszlo.szabo@phys.ubbcluj.ro Abstract Metal ions determination represents an area of interest in several fields, like environmental protection, food safety or clinical diagnostics. Analytical methodologies for direct determination of metal ions were established over the last decades

including atomic absorption or emission spectroscopy and mass spectrometry. Although these methods are sensitive and accurate, they require tedious sample pre-treatment and expensive equipment. Thus, a significant increase in the

development of optical chemical sensors for heavy metals has been noted in the last years, aiming to their routinely production, low cost, high selectivity and sensitivity. In this work, IR, Raman and SERS spectroscopy is used in conjunction with

quantum chemical calculations in order to characterize the molecular structure, electronic properties and vibrational energies of the 1-(2-pyridylazo)-2-naphthol (PAN) molecule and its complexes with Al(III), Ca(II), Mn(II), Fe(III), Cu(II), Zn(II) and

Pb(II). Thus, IR, Raman and SERS spectra of PAN, as well as the SERS spectrum of the Al(III), Ca(II), Mn(II), Fe(III), Cu(II), Zn(II) and Pb(II) complex of PAN, were assigned using DFT calculations with the hybrid B3LYP exchange-correlation

functional using the standard 6-31G(d) basis set. The experimental vibrational bands were assigned to the calculated normal modes and a very good correlation was achieved between the experimental and theoretical data. The SERS spectra of

PAN and of its metal chelates were recorded using HeNe laser emitting at 633 nm and hydroxylamine reduced silver colloid [1]. PAN complexes with Al(III), Ca(II), Mn(II), Fe(III), Cu(II), Zn(II) and Pb(II) are differentiated by their SERS spectra,

each metal complex showing a particular SERS spectral fingerprint. The molecular electrostatic potential of the molecules has been calculated and used for predicting site candidates of electrophilic attack.

Key words: Metal chelating compounds, heavy metal ion, FTIR, FT-Raman, SERS, DFT, 1-(2-pyridylazo)-2-naphthol (PAN).

Acknowledgement: This work was supported by CNCSIS –UEFISCSU, project number PNII – RU PD_445/2010.

Experimental techniques FT-IR /ATR The FTIR/ATR spectrum of PAN powder sample was recorded at room temperature on a conventional Equinox 55 (Bruker Optik GmbH, Ettlingen, Germany) FTIR spectrometer equipped with a DTGS detector. FT-Raman The FT-Raman spectrum of PAN was recorded in a backscattering geometry with a Bruker FRA 106/S Raman accessory equipped with a nitrogen cooled Ge detector. The 1064 nm Nd:YAG laser was used as excitation source, and the laser power measured at the sample position was 300 mW. The FT-Raman and FTIR/ATR spectra were recorded with a resolution of 4 cm-1 by co-adding 32 scans. SERS SERS spectra were recorded using a DeltaNu Advantage 633 Raman spectrometer (DeltaNu, Laramie, WY) equipped with a HeNe laser emitting at 633 nm. The laser power was 4 mW and the spectral resolution of 10 cm-1. For all SERS measurements 25 µl of analyte were added to 0.5 ml silver colloid. All chemicals used were of analytical reagent grade. The silver colloid was prepared according to the previously reported procedure [1]. The pH value of the silver colloid, measured immediately after preparation, was found to be 8. PAN complexes with Al(III), Ca(II), Mn(II), Fe(III), Cu(II), Zn(II) and Pb(II) were prepared by adding 1 ml dilutions of 10-3 M metal salt solution to 2 ml 10-3 M PAN solution, up to obtaining finally 3 ml mixtures at 2:1 PAN:metal salt molar ratio, PAN chelating metal ions at 2:1 ratio.

[1] N. Leopold, B. Lendl, J. Phys. Chem. B 107 (2003) 5723. Computational methods DFT exchange-correlation functionals: B3LYP & BLYP, basis sets: “spectroscopic” 6-31G(d)

Optimized chemical structure of PAN with atom numbering scheme.

FTIR/ATR spectra of PAN

800 1000 1200 1400 1600

734 75

810 85

1586 1691

159215

129812

Wavenumber /cm-1

PAN_FT-IR/ATR

Calculated IR spectrum

SERS, FT-Raman and Calculated Raman spectra of PAN

480 60

1592 16

161916

781554

128112

9422369

463 501 529

553 60

1116 11

1217 12

1379 14

200 400 600 800 1000 1200 1400 1600

ityWavenumber (cm-1)

SERS_PAN

FT-Raman_PAN

Calculated Raman spectrum

Optimized chemical structure of Mn(PAN)2 complex

Conclusions PAN was investigated by experimental (FT-IR, FT-Raman and SERS) techniques in conjunction with DFT quantum chemical calculations. In order to assess the detection potential of different metal ions PAN as chelating agent and SERS as detection method, SERS spectra of

different metal complexes of this ionophores were recorded (Al(III), Ca(II), Mn(II), Fe(III), Cu(II), Zn(II), Pb(II)) using a silver colloid substratum. Each PAN-metal complex SERS spectrum shows a characteristic spectral fingerprint. Because all PAN-metal compounds indicated standard

marker bands, SERS method may be a new detection technique for Al(III), Ca(II), Mn(II), Fe(III), Cu(II), Zn(II) and Pb(II). For PAN, DFT calculation were made for a good geometric optimization, for the molecular electrostatic potential determination and also for

band assignment.

B3LYP/6-31G(d) calculated 3D electrostatic potential contour map of PAN, in atomic units.

1051 10

94 1154 12

26 1256

409 45

1562 1590

476 588

758 85

1450 14

603 654

679 73

1326 13

1449 14

644 75

1560 1606

509 58

1508 15

60 1606

855 98

1093 1154

477 55

856 1000

200 400 600 800 1000 1200 1400 1600

Raman shift /cm-1

SERS-Al(PAN)2

SERS-Ca(PAN)2

SERS-Pb(PAN)2

SERS-Zn(PAN)2

SERS-Mn(PAN)2

SERS-Fe(PAN)2

SERS-Cu(PAN)2

SERS-PAN

SERS spectra of PAN and PAN-metal complexes

1165 12

43 1266

1529 15

221 26

463 52

3 608 648

911 96

1116 1139

1217 12

1379 14

526 572

1296 13

1550 15

416 467 57

885 933

1075 11

1446 1497

782 99

1398 14

1536 15

879 98

1274 13

11 1350

510 576

1277 13

07 1353

200 400 600 800 1000 1200 1400 1600

Pb(PAN)2

Cu(PAN)2

Fe(PAN)2

Mn(PAN)2

Ca(PAN)2

Al(PAN)2

Zn(PAN)2

Raman shift /cm-1

Calculated Raman spectra of PAN

and PAN-metal complexes B3LYP/6-31G(d) calculated 3D

electrostatic potential contour map of Mn(PAN)2 complex, in atomic units.

Picture of solutions of different PAN-metal complexes: PAN Al(PAN)2 Ca(PAN)2 Mn(PAN)2 Fe(PAN)2 Cu(PAN)2 Zn(PAN)2 Pb(PAN)2

SERS, Raman, IR and DFT investigation of 1-(2-pyridylazo ...

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