The Raman spectrum of the indocyanine-type dye IR-820 has been assigned for both solid and solution. SERS spectra of IR-820 on both silver and gold nanoparticles suspensions excited at 1064 nm were obtained. AgNPs allowed the detection of the dye through SERS down to 0.1 micromoles per liter; for the AuNPs the lowest concentration of the dye detectable was 10 micromoles per liter. Changes in the SERS relative intensities compared to the Raman spectrum in solution are subtle, mostly due to the preresonance effect of the dye. However, a perpendicular orientation relative to the metallic surface was inferred for the dye on both AgNPs and AuNPs. The easily distinguishable SERS spectra of the dye excited at 1064 nm, together with the high biological compatibility of cyanine dyes, are both indicative that IR-820 could be used as a high-performance probe molecule for SERS.
The surface-enhanced Raman scattering (SERS) effect has been studied for almost 40 years [
In addition to the dependence on the metallic nanoparticle characteristics, SERS is a strongly molecule and wavelength dependent effect, with the efficiency of the enhancement effect ranging over several orders of magnitude [
The use of near-infrared (NIR) excitation in SERS studies of biological systems is of great importance, as it is usual to observe intense fluorescence background contributions from components of the system itself when the excitation occurs in the visible range of frequency. This being said, the use of FT-Raman is attractive for such measurements with excitations at 1064 nm. However, there have only been a small number of works reporting the use of excitation at 1064 nm for SERS [
Taking into account the properties of cyanine-dyes, the IR-820 dye (structure presented in Scheme
(a) Schematic chemical structure of IR-820; the rings are labeled A–G. (b) Schematic structure of IR-820 optimized using DFT with the hybrid functional B3LYP and 6-311G(d,p) basis-set. The grey spheres represent C, blue for N, red for O, yellow for S, green for Cl, and purple for Na. (c) MEP maps for IR-820 calculated at the same level of theory as for (b); red-to-blue color scheme indicates negative-to-positive partial charges.
All chemicals were purchased from Sigma-Aldrich and IR-820, HAuCl4, AgNO3, and sodium citrate were of high purity and were used without further purification. Deionized water (
UV-visible spectra were acquired with a Shimadzu UVPC1000 spectrometer, using a 10 mm fused silica cell. The FTIR spectrum was acquired from the solid IR-820 dispersed in KBr pellet with a Bomem MB100 spectrometer. The Raman spectra have been acquired with a Bruker RFS 100 spectrometer and a Nd3+/YAG laser with a line at 1064 nm and with a 4 cm−1 spectral resolution, equipped with a Ge detector cooled with liquid nitrogen. The SERS spectra were acquired using different concentrations of IR-820: 0.1, 1.0, 10.0, and 100
The structure and vibrational frequencies of IR-820 have been calculated using the Gaussian09 suite of programs [
The UV-VIS spectrum of an aqueous solution of IR-820 is presented in Figure
UV-VIS spectrum of an aqueous solution of 10
The potential use of IR-820 as a high-performance SERS probe requires an assignment of the Raman and infrared spectra of the dye. The Raman and infrared spectra of solid IR-820 are presented in Figure
Band wavenumber (in cm−1) assignment for Raman, SERS (on Au and Ag nanoparticles), and infrared spectra of IR-820.
Theoretical/cm−1 | Infrared |
Raman solid/cm−1 | Raman solution/cm−1 | SERS |
SERS |
Assignment*,** |
---|---|---|---|---|---|---|
308 | — | 299 | 300 | 300 | 298 |
|
359 | — | — | 353 | 353 | 351 |
|
392 | — | — | 371 | 374 | 370 |
|
461 | 462 | 450 | 449 | 451 | 452 |
|
481 | 510 | 503 | 474 | 476 | 478 |
|
520 | — | 515 | 515 | 514 | 511 |
|
595 | — | 588 | — | 590 | 589 |
|
639 | 618 | — | — | — | — |
|
654 | — | — | 646 | 647 | 645 |
|
691 | 672 | 674 | 675 | 675 | 676 |
|
718 | — | 702 | 701 | 701 | 699 |
|
741 | 722 | 725 | 725 | 726 | 724 |
|
768 | — | 751 | 751 | 751 | 751 |
|
802 | 786 | 790 | 789 | 792 | 790 |
|
815 | 811 | 811 | — | — | — |
|
840 | 837 | 837 | 837 | 838 | 837 |
|
874 | 860 | 862 | 863 | 863 | 863 |
|
911 | 893 | 894 | 897 | 897 | 897 |
|
943 | 934 | 940 | 940 | 944 | 941 |
|
1011 | 1012 | 1011 | 1016 | 1014 | 1015 |
|
1046 | 1051 | 1059 | — | 1054 | — |
|
1072 | — | — | 1064 | 1067 | 1063 |
|
1161 | 1118 | 1114 | 1122 | 1122 | 1122 |
|
1174 | 1141 | 1146 | 1147 | 1148 | 1144 |
|
1208 | 1171 | 1166 | 1171 | 1167 | 1169 |
|
1223 | — | 1205 | 1209 | 1207 | 1205 |
|
1236 | 1237 | 1235 | 1238 | 1235 | 1236 |
|
1274 | 1273 | 1264 | 1265 | 1265 | 1263 |
|
1375 | 1375 | 1361 | 1361 | 1364 | 1363 |
|
1392 | 1392 | 1397 | 1399 | 1398 | 1400 |
|
1463 | — | — | 1415 | 1414 | 1415 |
|
1482 | 1446 | 1446 | 1447 | 1445 | 1446 |
|
1504 | 1463 | 1462 | 1464 | 1465 | 1463 |
|
1562 | 1523 | 1521 | 1522 | 1522 | 1523 |
|
1573 | 1549 | — | — | — | — |
|
1603 | 1575 | 1576 | 1575 | 1574 |
|
|
1640 | — | 1595 | 1595 | 1596 | 1596 |
|
1666 | 1626 | 1625 | 1626 | 1625 | 1626 |
|
Experimental spectra of solid IR-820: (a) FT-Raman and (b) FTIR spectra.
DFT calculated spectra of IR-820: (a) Raman and (b) infrared.
Table
The CCC bending of the conjugated central chain of IR-820 (see Scheme
The modes that present contributions from different rings of the IR-820 dye have been each labelled by letters from A to G in Table
Finally, the band at 481 cm−1 has been assigned to the symmetric bending of the
Figures
SERS spectra of IR-820 (a) at two solution concentrations on AgNP: (ii) 1
The SERS spectral profile of IR-820 on both AgNPs and AuNPs is somewhat similar to that reported for the indocyanine green (ICG) dye [
The SERS spectra of IR-820 on both AgNPs and AuNPs have also been compared to the Raman spectra of the dye in solution, also presented in Figure
Compared to the Raman spectra of IR-820 in solution, the bands at 450 and 1580 cm−1 present an increase in the relative intensity in the SERS spectra. Several other bands present changes in the relative intensity of the SERS spectra, taking the band at 1522 cm−1 as an example. It can also be observed that, for the bands at 943 and 1121 cm−1, no shoulder or low intensity band is found in the SERS spectra, unlike what has been observed in the Raman spectrum in solution. In addition, the relative intensities of the bands at 943, 1121, and 1522 cm−1 changed considerably in the SERS spectra. The observation of the mentioned changes in SERS relative intensity results in a dependence of the SERS enhancement factor for IR-820 on the vibrational mode, as it will be discussed later in this work.
The changes in the SERS spectra compared to the Raman spectra are small, as has been demonstrated above. Although not conclusive, this result is indicative of a small contribution of the chemical mechanisms for the SERS effect in the case of IR-820 [
However, there are not many changes between the spectra of IR-820 in solution and the SERS relative intensities, which could be a result of the frequency proximity of the IR-820 absorption band at 690 nm (
One interesting point resulting from the above characteristic of the SERS spectrum is that it is possible to directly calculate enhancement factors for IR-820 using the Raman spectrum as a reference, as the adsorbed chemical species is very similar to the solution species.
In order to evaluate the SERS spectra of IR-820 it is important to calculate the SERS enhancement factor (SERS-EF). Several methodologies for the SERS-EF calculation have been reported in the literature each of them being adequate for different types of SERS substrates and levels of knowledge of surface behavior of the adsorbate [
It should be noted that (
Analytical SERS-EF (AEF) for different concentrations of IR-820 on AgNPs and AuNPs calculated for three different bands.
Metal | IR-820 concentration ( |
SERS wavenumber (cm−1) | AEF |
---|---|---|---|
Ag | |
944 | 33 |
1122 | 23 | ||
1522 | 30 | ||
|
944 | 271 | |
1122 | 173 | ||
1522 | 257 | ||
|
944 | 2764 | |
1122 | 1809 | ||
1522 | 2810 | ||
|
944 | 4270 | |
1122 | 2915 | ||
1522 | 4013 | ||
|
|||
Au | |
941 | 11 |
1122 | 9 | ||
1523 | 9 | ||
|
941 | 63 | |
1122 | 39 | ||
1523 | 86 |
Another important observation from Table
The effect that the concentration of the adsorbate has on the SERS-EF decreases as the bulk concentration decreases, which results in an increase of the SERS-EF calculated with the AEF methodology [
Figure
SERS spectra of 1.0
The SERS-EFs also present a slight dependence on the vibrational mode, as it can be observed from the changes in the AEF calculated for the bands at 943, 1121, and 1522 cm−1 in Table
The indocyanine-type dye IR-820 had the vibrational Raman and FTIR spectra assigned with the support of DFT calculations. The vibrational assignment was used in order to better understand the SERS spectra of the dye on AgNPs and AuNPs suspensions. The SERS relative intensities only presented small changes when compared to the Raman spectra of the dye in solution. Additionally, the SERS relative intensities presented a weak dependence on both the surface material and vibrational mode, which indicated weak surface interaction. The weak dependence of SERS relative intensities on the vibrational modes, however, indicated that the dye was adsorbed with the aromatic rings perpendicular to the surface.
The weak dependence of the SERS relative intensities on the metallic surface and on the vibrational modes indicates that IR-820 is an interesting candidate as a probe molecule for SERS with excitation in the near infrared. The ubiquitous use of FT-Raman spectroscopy also indicates that IR-820 may be an interesting probe for SERS on such configurations.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank FAPEMIG, CNPq, UFJF, and “Rede Mineira de Química” (RQ-MG-FAPEMIG) for financial support. Gustavo F. S. Andrade thanks CNPq for a research fellowship. Tatiana B. V. Neves thanks CAPES for the grant of a fellowship.