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SERS, Raman, IR and DFT investigation of 1-(2-pyridylazo)-2-naphthol and its metal complexes L. Szabó 1 *, K. Herman 1 , N. Leopold 1 , A. Fălămaş 1 , N. Mircescu 1 , C. Buzumurgă 2 , V. Chiş 1 1 Faculty of Physics, Babeş-Bolyai University, Kogalniceanu 1, 400084 Cluj-Napoca, Romania 2 “Nicolae Stăncioiu" Heart Institute, Moţilor 19-21, 400001 Cluj-Napoca, Romania *[email protected] 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 648 670 734 756 786 810 858 873 911 964 985 1034 1066 1093 1116 1136 1191 1218 1252 1279 1313 1379 1408 1457 1467 1496 1524 1586 1691 1620 1606 1592 1568 1554 1502 1472 1453 1438 1402 1334 1320 1298 1262 1253 1230 1201 1157 1144 1135 1109 1098 1091 1045 1036 989 984 954 867 841 796 770 750 728 690 657 Absorbance (a.u.) Wavenumber /cm -1 PAN_FT-IR/ATR Calculated IR spectrum SERS, FT-Raman and Calculated Raman spectra of PAN 381 480 603 655 679 728 754 855 877 908 984 1002 1016 1050 1092 1153 1181 1229 1255 1310 1330 1357 1386 1433 1449 1477 1550 1574 1592 1609 1714 1619 1604 1592 1578 1554 1512 1482 1453 1441 1396 1353 1333 1321 1299 1281 1262 1252 1228 1201 1100 1090 984 906 896 856 784 761 727 706 644 600 586 569 551 539 522 499 460 449 422 369 349 315 302 252 223 261 317 354 425 463 501 529 553 608 648 734 779 810 859 911 964 1034 1116 1140 1217 1280 1379 1437 1496 1523 1550 1585 1615 1691 200 400 600 800 1000 1200 1400 1600 Raman Intensity Wavenumber (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. 236 382 481 606 660 730 856 985 1051 1094 1154 1226 1256 1310 1333 1388 1434 1479 1551 1574 1594 1611 409 454 513 538 570 649 761 856 912 1022 1096 1142 1232 1254 1334 1361 1454 1562 1590 1612 234 476 588 644 729 758 856 909 984 1016 1093 1154 1229 1255 1330 1356 1450 1478 1551 1594 455 482 603 654 679 730 760 810 856 911 984 1017 1094 1155 1232 1252 1326 1360 1449 1477 1551 1605 454 506 568 644 756 859 908 1016 1092 1138 1231 1329 1354 1449 1511 1560 1606 452 509 586 645 758 856 909 1018 1093 1140 1230 1326 1358 1450 1508 1560 1606 238 480 604 645 729 855 984 1016 1093 1154 1229 1255 1330 1357 1385 1450 1478 1551 1605 445 477 559 587 638 729 757 856 1000 1092 1154 1230 1255 1327 1447 1478 1551 1592 200 400 600 800 1000 1200 1400 1600 Raman Intensity 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 232 276 320 367 403 426 452 518 548 596 663 697 722 754 777 879 909 929 964 986 1007 1165 1207 1243 1266 1312 1399 1424 1466 1499 1529 1574 221 261 317 354 425 463 529 553 608 648 734 779 810 858 911 964 1034 1116 1139 1217 1280 1379 1437 1496 1523 1585 1691 211 250 296 364 445 526 572 649 671 876 975 1033 1096 1127 1232 1262 1296 1343 1434 1550 1593 263 325 353 416 467 575 615 677 783 885 933 1007 1075 1150 1205 1232 1289 1326 1383 1446 1497 1532 1575 200 237 270 300 324 516 569 606 782 996 1101 1135 1199 1260 1322 1398 1437 1536 1590 197 220 271 316 378 440 533 578 656 672 749 773 879 983 1029 1096 1125 1168 1238 1274 1311 1350 1438 1552 1591 229 309 368 446 510 576 652 673 771 878 981 1041 1096 1126 1166 1230 1277 1307 1353 1440 1469 1556 1594 224 294 378 437 497 519 568 627 649 668 878 973 1000 1032 1099 1124 1149 1232 1271 1297 1338 1436 1540 1594 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 PAN Raman Intensity 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
