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Raman and Computational Study on the Adsorption of Xanthine on
Silver Nanocolloids
Francesco Muniz-Miranda,*,†,∥ Alfonso Pedone,† and Maurizio Muniz-Miranda‡,§
†
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Department of Chemical and Geological Sciences (DSCG), University of Modena and Reggio Emilia (UniMORE), Via Campi 103,
41125 Modena, Italy
‡
Department of Chemistry “Ugo Schiff”, University of Florence (UniFI), Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy
§
Institute of Complex Systems (CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
S Supporting Information
*
ABSTRACT: Xanthine is a nucleobase, deriving from
adenine and guanine by deamination and oxidation processes,
which may deposit in the human body causing diseases,
similar to uric acid. Here, we have investigated the adsorption
of xanthine on silver colloidal nanoparticles by means of
surface-enhanced Raman scattering (SERS) with an exciting
radiation in the near-infrared spectral region, where
interference due to fluorescence does not occur, along with
density functional theory calculations of molecule/metal
model systems. By adopting a combined experimental and
computational approach, we have identified the “marker”
SERS bands of xanthine and the tautomer that preferentially
binds the silver particles, as well as the molecular group
involved in the interaction with metal. This investigation allows using the FT-SERS spectroscopy for biosensory and diagnostic
purposes in body fluids, detecting abnormal levels of xanthine, and preventing metabolic diseases.
(SERS) spectroscopy7,8 represents a valid alternative to the
HPLC technique. SERS allows obtaining a trace detection of
different molecules when the latter are adsorbed on nanostructured surfaces of noble metals as silver or gold. Actually,
unlike the normal Raman scattering that has scarce sensitivity
due to its small cross section, SERS ensures huge enhancements of the spectral signal of the adsorbed molecules, with
factors generally up to 106−107 with respect to the Raman
response due to nonadsorbed ones but up to 1014−1015 factors
in single-molecule experiments.9,10 However, the validity of
this spectroscopic technique is not only and not so much due
to the sensitivity of the technique but (1) in the easy molecular
recognition of the “markers” bands of the molecule and (2) in
the simplicity of the investigation procedures that do not need
any sample manipulation or separation procedure.
Here, we study the SERS response of xanthine adsorbed on
silver colloidal nanoparticles, in view of analytical applications
in biomedicine. For this purpose, we employed the FT-Raman
instrumentation, with excitation in the near-infrared (NIR)
spectral region. This excitation, despite the low intensity of the
Raman response in the NIR region (owing to the λ−4
dependence of Raman scattering intensity), is highly desirable
for all of these applications requiring removal of any
INTRODUCTION
The ability to recognize and quantitatively evaluate nucleotides, nucleosides, and their bases assumes great relevance in
biomedical applications,1−3 mainly because it allows to easily
monitor the degradation processes of nucleic acids present in
body fluids and tissues. In fact, concentration changes of these
components may pinpoint alterations in the physiological
activity and presence of disease states. Xanthine is a purine
base that is not commonly present in RNA or DNA chains yet
derives from adenine and guanine by deamination and
oxidation processes, as illustrated in Figure 1. Also, xanthine
undergoes oxidation to uric acid, as shown in Figure 1, by the
action of the xanthine oxidase enzyme.
As a consequence, a level of xanthine in the human body
higher than normal increases the presence of uric acid, forming
kidney stones with serious problems to the entire urinary tract.
Moreover, xanthine, similar to uric acid, may deposit in the
human body and cause diseases due to its scarce solubility in
water.4,5 Hence, it is important to have a reliable and sensitive
technique for the identification of xanthine in the human
organism and prevent health-threatening conditions. In
particular, the recent developments in high-performance liquid
chromatography (HPLC) allow the simultaneous determination of purine metabolites as xanthine, hypoxanthine, and
uric acid in the human plasma and serum.6 However, the
possibility of evidencing the presence of xanthine in biological
liquids through the surface-enhanced Raman scattering
■
© 2018 American Chemical Society
Received: August 26, 2018
Accepted: October 8, 2018
Published: October 18, 2018
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Figure 1. Formation of xanthine and uric acid from adenine and guanine.
interference of fluorescence from the Raman response. In fact,
fluorescence emission can be due to impurities that, even in
traces, add high levels of shot noise on the Raman bands,
making their observation a difficult task, as often occurring in
biological samples.11 Moreover, the NIR region being
coincident with the so-called biological tissue transparency
window,12 this could permit the use of the SERS spectroscopy
to recognize the presence of xanthine both in vitro and in vivo.
The interpretation of the SERS data can be efficiently
performed by means of density functional theory (DFT)
calculations based on model systems constituted of molecules
bound to adatoms or adclusters with a few metal atoms, which
are able to simulate the active sites present on the surface of
the colloidal nanoparticles.13−16 This method allows not only
to reproduce accurately the observed SERS profiles, including
band positions and relative intensities, but also to understand
the type and strength of the molecule/metal interactions.
Moreover, the DFT approach showed to be particularly useful
in identifying the molecular species that chemically interact
with metal.17−19 This information is essential to identify and
quantify the presence of xanthine in a mixture of different
compounds, as expected in body fluids, as well as to design
analytical protocols to perform such analysis.
Figure 2. Di-keto tautomeric forms of xanthine.
respect to those with enolic structures, on the basis of the
energies estimated by AM1-PM3 study21 and DFT calculations;20,22,23 these latter, moreover, indicated the N(7)H
tautomer as more stable than the N(9)H tautomer. However,
in acidic-neutral water solutions, where xanthine is not
deprotonated, both tautomers are expected to be present.24
We have obtained FT-Raman spectra of xanthine in aqueous
solutions at different pH values, as shown in Figure 3. It is
important to show these spectra to observe possible similarities
with the SERS spectrum of xanthine, which could suggest
which molecular species is really adsorbed on silver. Moreover,
the Raman spectra of xanthine in solid phase (see Figure 3),
where the presence of the N(7)H tautomer is expected, can
provide information on the possible adsorption of this species
on silver.
Sizeable Raman differences occur in aqueous solutions with
respect to the Raman spectrum of the solid sample and also in
solution by varying the pH values. At pH = 6 value, where
xanthine, having pKa = 7.60, is predominantly present as
RESULTS
Structure and FT-Raman Spectra of Xanthine. As
xanthine can exist as many possible tautomers,20 knowing the
precise tautomers of this nucleobase is crucial to understand
the mechanisms of the enzymes of which it is a substrate.
However, the di-ketoforms, the N(9)H and N(7)H tautomers
(see Figure 2), were considered by far the most probable with
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the pH is slightly lower (pH ∼ 6), because we waited a few
weeks after the colloid preparation before using the Ag
suspension to obtain a better stabilization.
After addition of xanthine (10−6 M concentration) to the
citrate-reduced Ag colloid, the maximum of the plasmon band
of the silver nanoparticles occurs at 425 nm wavelength, with a
tail in the red-light region (see Figure 4). In this situation, no
Figure 3. Normal Raman spectra of xanthine as a solid sample and
dissolved in water solutions at different pH values, in comparison with
the corresponding SERS spectrum in Ag colloid. Concentrations: 10−1
M (water, pH 12), 10−3 M (water, pH 6), and 10−6 M (Ag colloid).
Excitation: 1064 nm.
neutral molecule, the Raman spectrum is quite weak, because
neutral xanthine is scarcely soluble in water. At pH = 12,
instead, the Raman spectrum of the deprotonated xanthine is
quite intense, because high concentrations (around 10−1 M)
can be obtained in water. However, the strongest band occurs
in all spectra around 650 cm−1; this band, for which position
and intensity do not significantly change by varying the
physical state and the pH value in solution, can be confidently
attributed to the breathing mode of the ring system of
xanthine. The band observed at 1680 cm−1 in the solid Raman
spectrum is due to CO stretching mode, with corresponding
bands occurring around 1695 cm−1 in the aqueous solutions.
The observed frequencies are reported in Table S1 of the
Supporting Information; the solid Raman frequencies observed
in the present study are similar to those previously reported in
2004 and 200525,26 but differ from some bands reported in
2011,27 in both positions and relative intensities. This evidence
could be attributed to a lower purity degree of the sample
examined in 2011.
Adsorption of Xanthine on Silver Colloidal Nanoparticles. The aim of this paper, devoted to the SERS
detection of xanthine, is to use a metal colloidal suspension
that exhibits these two characteristics: colloidal stability and
SERS efficiency. Therefore, we chose to use a silver colloid
obtained by reduction of silver ions with citrate, according to
the well-tested procedure by Lee and Meisel,28 because it
exhibits a marked stability in both absence and presence of
organic ligand, thanks to the protection ensured by adsorbed
citrate anions. This is particularly important for the
identification and quantitative determination of xanthine,
since colloidal aggregation processes that evolve in time
could significantly alter the SERS response.
Several studies were performed to characterize the silver
colloids obtained by reduction with citrate according to Lee
and Meisel.28 It is important to point out that boiling the
colloidal suspension caused silver nanoparticles to be
monodispersed by the capping effect of the citrate anions,
with maximum absorption of the plasmonic band around 420
nm.28 The resulting pH value was around 6.5.29 In our case,
Figure 4. UV−visible extinction spectra of a silver colloid with 10−6
M xanthine, before (A) and after (B) addition of 10−3 M NaCl. The
1064 nm laser line of the FT-Raman instrument is indicated.
FT-SERS signal can be detected by laser excitation at 1064 nm,
because the excitation wavelength does not match the plasmon
band. However, by adding NaCl (10−3 M concentration), the
plasmon band markedly moves to the infrared region, due to
the aggregation of the silver particles promoted by the
presence of co-adsorbed chloride anions. In this case, strong
FT-SERS bands can be observed. The SERS activation of the
silver colloids by excitation in the NIR region was previously
ascertained and discussed by Kiefer and co-workers.30−32
FT-SERS Spectra of Xanthine in Silver Colloids. Figure
3 (upper panel) shows the FT-SERS spectrum of xanthine
adsorbed on Ag colloid, which appears dominated by a very
strong band at 657 cm−1, similar to what observed in the
normal Raman spectra of xanthine in solutions and in solid
phase. The SERS spectrum is quite different, in both band
frequencies and intensities, from the very weak FT-SERS
spectrum previously reported;33 the band positions, instead,
appear similar to those obtained by Cotton and co-workers by
visible excitation.34 At the pH value of the Ag colloid (∼pH =
6), xanthine is expected to adsorb on silver as a neutral
molecule, which is predominantly present in aqueous solution
at this pH value. This, however, does not mean that the
molecule in silver colloidal suspension at this pH value cannot
bind to the metal as a deprotonated anion. In fact, when a
molecule chemically interacts with silver colloidal nanoparticles, deprotonation reactions may occur by effect of the
metal surface, although the deprotonated species is absolutely
negligible at the pH value of the aqueous solution. In our case,
however, the strong Raman bands of deprotonated xanthine
observed at 1273, 1352, and 1518 cm−1 in the Raman
spectrum of the alkaline solution (see Figure 3) do not find
any correspondence in the SERS spectrum. This leads to
exclude an adsorption of xanthine in anionic form. Moreover,
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Figure 5. DFT-optimized (B3LYP/def2TZVPP) structures of the xanthine/silver complexes.
chemical mechanism. Regarding the first one, the molecules
approaching a noble-metal nanoparticle undergo an electric
field several orders of magnitude (at least four) larger than that
at long distances from the surface, under the action of the
electromagnetic radiation. The second contribution to the
SERS enhancement, instead, depends on the change of the
molecular polarizability due to the formation of chemical
bonds with the active sites of the metal surface. Although the
electromagnetic contribution plays a predominant role for the
overall SERS enhancement, the chemical one is important to
determine the SERS spectral pattern, concerning both spectral
positions and relative intensities of the observed bands. Hence,
for chemisorbed molecules, the profiles of the SERS spectra are
essentially determined by the chemical interactions between
the molecules and the active sites of the metal surface, called
adatoms, constituted by one or a few atoms, as stated by
Otto36 and Aroca.37 The validity of the adatom approximation
was widely verified by DFT calculations of many adsorbed
molecules; in particular, the DFT-calculated Raman spectra of
molecule/metal complexes with a single silver atom are quite
often able to reproduce satisfactorily the corresponding SERS
spectra, as shown also very recently.38−40 It is important to
underline that the adatoms present on the surface of the silver
colloidal nanoparticles can be effectively considered positively
charged.12−14 In this regard, the adsorption of the negatively
charged citrate ions on the silver colloidal surface plays an
important role: the citrate anions not only perform a stabilizing
action of the colloidal suspension but also promote the
formation of positive charges on the silver surface.
For our DFT calculations on the complexes of xanthine
linked to one Ag ion, we used the hybrid functional B3LYP in
combination with two different basis sets: def2TZVPP and a
mixed basis set 6-311G++(d,p) for all atoms except silver,
treated with Lanl2DZ. Table S2 (Supporting Information)
reports the energies of the complexes calculated with the
def2TVZPP basis set. The N(9)H-A complex, where the silver
adatom is linked to the N(9)H tautomer in a bidentate way, is
by far the most stable, whereas the N(9)H-B complex exhibits
marked instability. The complexes with the N(7)H tautomer
present similar energies. Moreover, the N(9)H-A complex
provides the largest molecule → metal charge transfer (Table
the SERS spectrum appear quite different also from the normal
Raman spectrum of the solid sample (see Table S1 in the
Supporting Information), where the molecule is present as
N(7)H tautomer, suggesting this latter is not involved in the
interaction with metal. Spectral differences, however, occur in
the SERS spectrum also with respect to the normal Raman
spectrum of the aqueous solution at pH = 6, where the
coexistence of both tautomers is expected. Besides marked
frequency shifts observed in the 1200−1700 cm−1 spectral
region, the highest-frequency band occurs in the SERS
spectrum beyond 1700 cm−1, unlike in the normal Raman
spectra. The FT-SERS spectrum of xanthine in Ag colloid is
similar to that of guanine adsorbed on silver,35 especially by
considering the dominant band around 650 cm−1 and the highfrequency band at about 1710 cm−1.
The spectral differences between SERS and Raman spectra
of xanthine are attributable to the interaction with silver, which
is to be interpreted as chemisorption, which significantly
modifies the force constants of the molecule as well as their
relative Raman intensities. For this reason, the DFT approach
could provide important information on the adsorption of
xanthine, concerning the preferential tautomer interacting with
metal, the molecular site of interaction with the active sites of
the Ag nanoparticles, and also the nature of the latter. We have
adopted five different complexes of both tautomers of xanthine
linked to silver nanoparticles through the nitrogen atoms with
sp2 hybridization or the oxygen atoms of the carbonyl groups.
The metal active sites are simply modeled as Ag+ adatoms, as
previously carried out for adsorbed adenine.14 The DFToptimized structures of these complexes are shown in Figure 5.
The N(9)H tautomer can link the silver adatom via N7 or
O10, in the N(9)H-A or N(9)H-B complexes, respectively. In
the first case, also the O11 atom of the CO group vicinal to
N7 is involved in the binding with the metal surface. The
tautomer N(7)H, instead, can link the silver adatom via N9,
O6, or O2, in the N(7)H-A, N(7)H-B, or N(7)H-C
complexes, respectively.
DFT Simulations of the SERS Spectrum of Xanthine in
Silver Colloid. SERS enhancement is generally considered7,8
as the product of two contributions, one due to a long-range
electromagnetic mechanism and another due to a short-range
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S2 in the Supporting Information), as obtained from the
Hirshfeld partial charges,41 reported in Table S3 (Supporting
Information) for the N(9)H-A complex.
As shown in Table 1, only the complex N(9)H-A is able to
reproduce satisfactorily all observed SERS frequencies, in
Table 1. Observed and Calculateda SERS Frequencies of
Xanthine
FT-SERS
Ag colloid
calcd
N(9)H-A
complex
calcd
N(9)H-B
complex
calcd
N(7)H-A
complex
calcd
N(7)H-B
complex
calcd
N(7)H-C
complex
516
575
657
870
957
1132
1223
1245
1320
1364
1380
1430
1476
1551
1573
1704
531
582
634
849
956
1150
1221
1268
1303
1351
1404
1430
1458
1529
1598
1703
530
539
623
858
950
533
627
857
961
1123
1214
1261
1305
502
597
656
845
956
1126
1224
1248
1330
1421
1458
1393
1454
1481
545
583
634
862
965
1122
1206
1283
1313
1374
1387
1457
1476
1593
1581
1574
1278
1339
1365
1398
1465
1521
1597
a
B3LYP/def2TZVPP.
particular, those occurring in the 1200−1550 cm−1 spectral
region and that around 1700 cm−1. Quite similar results are
obtained by using the mixed basis set, 6-311G++(d,p)/
Lanl2DZ. Regarding the higher-frequency bands, it can be
observed that the stretching vibrations of the CO bonds that
are not engaged in the interaction with metal are overestimated
by using both def2TZVPP and the mixed basis set, as
previously observed,42 because the interactions with the
molecules of the water solvent are not taken into account in
our calculation models. Actually, it was ascertained that the
CO stretching frequencies are largely overestimated when
the protic solvent is not considered in the DFT calculations.43−45
These computational results suggest that the preferential
adsorption of xanthine on silver occurs with the N(9)H
tautomer, by involving both the N7 atom and the oxygen atom
of the vicinal carbonyl group. This is confirmed by comparing
the observed SERS spectrum with that calculated with all
complexes, as shown in Figure 6 in the case of the def2TVZPP
basis set. Very similar results, by considering band frequencies
and intensities, are obtained with the mixed basis set, as shown
in Figure S1 (Supporting Information). Only the N(9)H-A
complex can simulate satisfactorily all experimental features,
regarding both positions and relative intensities of the SERS
bands. In particular, only this model is able to show the
dominance of the SERS band around 650 cm−1, as well as the
occurrence of the strong SERS band around 1700 cm−1.
Finally, Figure 7 shows the normal modes, calculated with
the def2TZVPP basis set, corresponding to the prominent
SERS bands, observed at 647, 957, 1245, 1320, and 1704 cm−1,
which may be considered as “marker” bands of xanthine
adsorbed on silver. All of these bands are assigned to in-plane
vibrations; in particular, the dominant band at 657 cm−1 is
Figure 6. DFT-simulated (B3LYP/def2TZVPP) SERS spectra of the
xanthine/silver complexes, compared with the observed SERS
spectrum of xanthine adsorbed on Ag nanoparticles.
attributable to the ring breathing vibration, that at 957 cm−1 is
attributable to deformation of the imidazolic ring, those at
1245 and 1320 cm−1 are attributable to ring deformations
mixed with N-H bending vibrations, whereas that at 1704 cm−1
is attributable to the stretching mode of the CO bond
involved in the binding with silver.
CONCLUDING REMARKS
Xanthine is a purine nucleobase that is not present in DNA/
RNA chains but derives from the metabolic degradation of
guanine and adenine, as described in Figure 1. Abnormal levels
of xanthine and uric acid (oxidation product of xanthine) in
the human body lead to the emergence of severe pathologies.
This work has shown how it is possible to use the SERS
spectroscopy in the early diagnosis of these diseases by
determining the presence of xanthine in body fluids. For this
aim, we have obtained a strong SERS activation in the NIR
region, where fluorescence phenomena that interfere with the
SERS signal do not usually occur. Moreover, the use of FTRaman spectroscopy can allow the SERS investigation both in
vitro and in vivo, given that the laser exciting radiation at λ =
1064 nm falls in the biological tissue transparency window. In
the present case, the strong FT-SERS activation is due to both
electromagnetic and chemical mechanisms: the first is due to
the shift of the plasmon band toward the infrared region; the
second one is due to the formation of charge-transfer
complexes induced by the presence of chloride anions.46
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Figure 7. Optimized geometry of the N(9)H-A complex with distances in angstroms, along with the normal modes corresponding to the
prominent SERS bands observed in the Ag colloid.
Raman Measurements. Raman spectra of xanthine as a
solid sample, dissolved in acidic-neutral (around pH = 6) and
alkaline (around pH = 12) aqueous solutions or adsorbed on
Ag colloidal nanoparticles, were recorded with a Fourier
transform (FT)-Raman spectrometer (Bruker Optics, Model
MultiRam), equipped with a broad-range quartz beamsplitter,
an air-cooled Nd:YAG laser excitation source (1064 nm), and
a Ge diode detector cooled with liquid nitrogen. The
instrument provided a spectral range of 3600−50 cm−1
(Stokes shift). The experiments were performed in a 180°
geometry, with 200 mW of laser power.
We have identified and assigned the marker bands in the
SERS spectrum of xanthine to detect this nucleobase in the
presence of other organic components. To do this, we have
used the DFT computational approach, as the interpretation of
the SERS spectrum of xanthine is complicated by the possible
presence of two tautomers and of different complexes formed
by molecule−metal interactions. In particular, the DFT
simulations, based both on the frequencies and relative
intensities of the calculated spectra and on the calculated
energies of the different complexes, have allowed: (i) to
recognize the tautomer N(9)H as the molecular species linked
to the silver nanoparticles, (ii) to identify the molecular site
involved in the interaction with silver, that is, the N7 atom
together with the vicinal carbonyl group, (iii) to highlight the
interaction of xanthine with silver, that is, a strong
chemisorption with a marked molecule → metal charge
transfer, and (iiii) to evidence the nature of the active sites of
the metallic surface, which are effectively modeled by positively
charged silver atoms.
■
COMPUTATIONAL DETAILS
Calculations were performed with the Gaussian 09 software.47
We adopted the hybrid B3LYP exchange-correlation functional,48,49 along two different basis sets: def2TZVPP50−52 and
a mixed basis set made up of 6-311++G(d,p) for nonmetal
atoms and Lanl2DZ53−58 for silver. Calculations consisted of
structural optimizations of the tautomers of xanthine linked to
metal adatoms, followed by vibrational frequency calculations.
We checked that all vibrational frequencies were real and
positive, thus belonging to geometries corresponding to true
potential energy minima.
The calculated Raman activities (Ai) were converted to
relative Raman intensities (Ii) using the following relationship,
as reported in the literature59,60
EXPERIMENTAL SECTION
Preparation of Silver Colloids. Stable Ag colloids were
prepared in triple distilled water by reducing silver nitrate with
sodium citrate at its boiling point, according to Lee and
Meisel.28 Sodium citrate dehydrate (Sigma, purity ≥ 99.5%)
was added dropwise to AgNO3 (Aldrich, purity 99.9999%)
aqueous solution under vigorous stirring; the presence of
citrate ensured long-term colloidal stability. Silver colloids,
used some weeks after the preparation, had a pH value around
6. Xanthine (purity ≥ 99.5%), supplied by Sigma as purified by
recrystallization, was added to the Ag colloidal suspension in a
10−6 M final concentration. To observe an FT-SERS signal of
xanthine, NaCl was added to the Ag colloid in a 10−3 M final
concentration.
UV−Visible Absorption Measurements. Absorption
spectra of the silver colloidal suspensions were observed in
the 200−1100 nm region by means of a Cary 5 Varian
spectrophotometer.
■
Ii =
f (νo − νi)4 A i
νi(1 − e−hcνi / kT )
where νo is the exciting frequency (in cm−1 units); νi is the
vibrational frequency (in cm−1 units) of the ith normal mode; h,
c, and k are fundamental constants; and f is a suitably chosen
common normalization factor for all peak intensities.
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(9) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.;
Dasari, R. R.; Feld, M. S. Single Molecule Detection Using SurfaceEnhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667−
1670.
(10) Nie, S.; Emory, S. R. Probing Single Molecules and Single
Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997,
275, 1102−1106.
(11) Chase, D. B. Fourier transform Raman spectroscopy. J. Am.
Chem. Soc. 1986, 108, 7485−7488.
(12) Smith, A. M.; Mancini, M. C.; Nie, S. Bioimaging: Second
window for in vivo imaging. Nat. Nanotechnol. 2009, 4, 710−711.
(13) Owen, A. R.; Golden, J. W.; Price, A. S.; Henry, W. A.; Barker,
W. R.; Perry, D. A. Surface-Enhanced Vibrational Spectroscopy and
Density Functional Theory Study of Isoniazid Layers Adsorbed on
Silver Nanostructures. J. Phys. Chem. C 2014, 118, 28959−28969.
(14) Pagliai, M.; Caporali, S.; Muniz-Miranda, M.; Pratesi, G.;
Schettino, V. SERS, XPS, and DFT Study of Adenine Adsorption on
Silver and Gold Surfaces. J. Phys. Chem. Lett. 2012, 3, 242−245.
(15) Muniz-Miranda, M.; Pagliai, M. Positively Charged Active Sites
for the Adsorption of Five-Membered Heterocycles on Silver
Colloids. J. Phys. Chem. C 2013, 117, 2328−2333.
(16) Muniz-Miranda, M.; Muniz-Miranda, F.; Pedone, A. Raman
and DFT study of methimazole chemisorbed on gold colloidal
nanoparticles. Phys. Chem. Chem. Phys. 2016, 18, 5974−5980.
(17) Maiti, N.; Thomas, S.; Debnath, A.; Kapoor, S. Raman and XPS
study on the interaction of taurine with silver nanoparticles. RSC Adv.
2016, 6, 56406−56411.
(18) Dhayagude, A. C.; Maiti, N.; Debnath, A. K.; Joshi, S. S.;
Kapoor, S. Metal nanoparticle catalyzed charge rearrangement in
selenourea probed by surface-enhanced Raman scattering. RSC Adv.
2016, 6, 17405−17414.
(19) Muniz-Miranda, M.; Muniz-Miranda, F.; Caporali, S. SERS and
DFT study of copper surfaces coated with corrosion inhibitor.
Beilstein J. Nanotechnol. 2014, 5, 2489−2497.
(20) Polat, T.; Yıldırım, G. Investigation of solvent polarity effect on
molecular structure and vibrational spectrum of xanthine with the aid
of quantum chemical computations. Spectrochim. Acta, Part A 2014,
123, 98−109.
(21) Civcir, P. Ü . AM1 and PM3 study of tautomerism of xanthine
in the gas and aqueous phases. J. Mol. Struct.: THEOCHEM 2001,
545, 7−15.
(22) Kim, J. H.; Odutola, J. A.; Popham, J.; Jones, L.; von Laven, S.
Tautomeric Energetics of Xanthine Oxidase Substrates: Xanthine, 2Oxo-6-methylpurine and Lumazine. J. Inorg. Biochem. 2001, 84, 145−
150.
(23) Gobre, V. V.; Pinjari, R. V.; Gejji, S. P. Structure and Normal
Vibrations in Xanthine and its Methyl derivatives from First Principle
Calculations. J. Mol. Struct.: THEOCHEM 2010, 960, 86−92.
(24) Kulikowska, E.; Kierdaszuk, B.; Shugar, D. Xanthine, xanthosine
and its nucleotides: solution structures of neutral and ionic forms, and
relevance to substrate properties in various enzyme systems and
metabolic pathways. Acta Biochim. Pol. 2004, 51, 493−531 DOI:
035001493 .
(25) Krishnakumar, M.; Arivazhagan, M. Vibrational and normal
coordinate analysis of xanthine and hypoxanthine. Indian J. Pure Appl.
Phys. 2004, 42, 411−418 IPC Code: GO1J 3/44 .
(26) Gunasekaran, S.; Sankari, G.; Ponnusamy, S. Vibrational
spectral investigation on xanthine and its derivativestheophylline,
caffeine and theobromine. Spectrochim. Acta, Part A 2005, 61, 117−
127.
(27) Arivazhagan, M.; Jeyavijayan, S. FTIR and FT-Raman spectra,
assignments, ab initio HF and DFT analysis of xanthine. Spectrochim.
Acta, Part A 2011, 79, 161−168.
(28) Lee, P. C.; Meisel, D. Adsorption and surface-enhanced Raman
of dyes on silver and gold sols. J. Phys. Chem. 1982, 86, 3391−3395.
(29) Cañamares, M. V.; Garcia-Ramos, J. V.; Gómez-Varga, J. D.;
Domingo, C.; Sanchez-Cortes, S. Comparative Study of the
Morphology, Aggregation, Adherence to Glass, and Surface-Enhanced
Raman Scattering Activity of Silver Nanoparticles Prepared by
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsomega.8b02174.
Observed Raman frequencies of xanthine; calculated
energies and molecule → metal electronic charge
transfers in silver complexes; Hirshfeld atomic charges;
comparison between the simulated SERS spectra of the
N(9)H-A complex, obtained with the 6-311G++(d,p)/
Lanl2DZ and def2TZVPP basis sets (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: francesco.munizmiranda@ugent.be, f.
munizmiranda@gmail.com.
ORCID
Alfonso Pedone: 0000-0003-3772-7222
Maurizio Muniz-Miranda: 0000-0001-9457-6833
Present Address
∥
Post-doctoral fellow at the Department of Physics and
Astronomy, Center for Molecular Modeling (CMM), Ghent
University (UGent), Technologiepark 903, 9052 Zwijnaarde,
Belgium (F.M.M.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
FM-M’s post-doctoral fellowship at UniMORE in Prof. Dr.
Alfonso Pedone’s group during the 2013−2017 period was
supported by the Italian “Ministero dell’Istruzione, dell’Università e della Ricerca” (MIUR) through the “Futuro in
Ricerca” (FIRB) Grant RBFR1248UI_002 entitled “Novel
Multiscale Theoretical/Computational Strategies for the
Design of Photo and Thermo responsive Hybrid Organic−
Inorganic Components for Nanoelectronic Circuits”.
■
■
REFERENCES
(1) Eichhorn, G. L. The effect of metal ions on the structure and
function of nucleic acids. In Advances in Inorganic Biochemistry,
Eichhorn, Gunther L., Marzilli, Luigi G., Eds.; Elsevier: New York,
1981, Vol. 3, pp. 1−46, ISBN-10: 0444006370.
(2) Azam, S.; Hadi, N.; Khan, N. U.; Hadi, S. M. Antioxidant and
prooxidant properties of caffeine, theobromine and xanthine. Med. Sci.
Monit. 2003, 9, BR325−330 PMID: 12960921 .
(3) Blackburn, G. M. Nucleic Acids in Chemistry and Biology, 3rd ed.;
RSC Pub.: Cambridge, 2006.
(4) Königsberger, E.; Wang, Z.; Seidel, J.; Wolf, G. Solubility and
dissolution enthalpy of xanthine. J. Chem. Thermodyn. 2001, 33, 1−9.
(5) Ichida, K.; Amaya, Y.; Kamatani, N.; Nishino, T.; Hosoya, T.;
Sakai, O. Identification of two mutations in human xanthine
dehydrogenase gene responsible for classical type I xanthinuria. J.
Clin. Invest. 1997, 99, 2391−2397.
(6) Pleskačová, A.; Brejcha, S.; Pácal, L.; Kaňková, K.; Tomandl, J.
Simultaneous Determination of Uric Acid, Xanthine and Hypoxanthine in Human Plasma and Serum by HPLC−UV: Uric Acid
Metabolism Tracking. Chromatographia 2017, 80, 529−536.
(7) Schlücker, S. Surface Enhanced Raman Spectroscopy: Analytical,
Biophysical and Life Science Applications; Wiley-VCH: Weinheim,
Germany, 2011.
(8) Procházka, M. Surface-Enhanced Raman Spectroscopy, Bioanalytical, Biomolecular and Medical Applications; Springer: Switzerland,
2016.
13536
DOI: 10.1021/acsomega.8b02174
ACS Omega 2018, 3, 13530−13537
ACS Omega
Article
Chemical Reduction of Ag+ Using Citrate and Hydroxylamine.
Langmuir 2005, 21, 8546−8553.
(30) Liang, E. J.; Engert, C.; Kiefer, W. Surface-enhanced Raman
scattering of pyridine in silver colloids excited in the near-infrared
region. J. Raman Spectrosc. 1993, 24, 775−779.
(31) Liang, E. J.; Kiefer, W. Chemical Effect of SERS with NearInfrared Excitation. J. Raman Spectrosc. 1996, 27, 879−885.
(32) Liang, E. J.; Ye, X. L.; Kiefer, W. Surface-Enhanced Raman
Spectroscopy of Crystal Violet in the Presence of Halide and Halate
Ions with Near-Infrared Wavelength Excitation. J. Phys. Chem. A
1997, 101, 7330−7335.
(33) Ranc, V.; Hruzikova, J.; Maitner, K.; Prucek, R.; Milde, D.;
Kvítek, L. Quantification of purine basis in their mixtures at femtomolar concentration levels using FT-SERS. J. Raman Spectrosc. 2012,
43, 971−976.
(34) Sheng, R.; Ni, F.; Cotton, T. M. Determination of purine bases
by reversed-phase high-performance liquid chromatography using
real-time surface-enhanced Raman spectroscopy. Anal. Chem. 1991,
63, 437−442.
(35) Giese, B.; McNaughton, D. Density functional theoretical
(DFT) and surface-enhanced Raman spectroscopic study of guanine
and its alkylated derivatives. Phys. Chem. Chem. Phys. 2002, 4, 5171−
5182.
(36) Otto, A. The ‘chemical’ (electronic) contribution to surfaceenhanced Raman scattering. J. Raman Spectrosc. 2005, 36, 497−509.
(37) Aroca, R. Surface-Enhanced Vibrational Spectroscopy; Wiley:
New York, 2006.
(38) Wrzosek, B.; Cukras, J.; Dobrowolski, M. A.; Bukowska, J. Real
Chemical States of the 3-Sulfur Derivative of 1,2,4-Triazole in
Different Conditions: Complex Experimental and Theoretical Studies.
J. Phys. Chem. C 2017, 121, 9282−9295.
(39) Harroun, S. G.; Zhang, Y.; Chen, T.-H.; Ku, C.-R.; Chang, H.T. Biomarkers of cigarette smoking and DNA methylating agents:
Raman, SERS and DFT study of 3-methyladenine and 7methyladenine. Spectrochim. Acta, Part A 2017, 176, 1−7.
(40) Al-Shalalfeh, M. M.; Saleh, T. A.; Al-Saadi, A. A. Silver colloid
and film substrates in surface-enhanced Raman scattering for 2thiouracil detection. RSC Adv. 2016, 6, 75282−75292.
(41) Saha, S.; Roy, R. K.; Ayers, P. W. Are the Hirshfeld and
Mulliken population analysis schemes consistent with chemical
intuition? Int. J. Quantum Chem. 2009, 109, 1790−1806.
(42) Gogia, S.; Jain, A.; Puranik, M. Structures, Ionization Equilibria,
and Tautomerism of 6-Oxopurines in Solution. J. Phys. Chem. B 2009,
113, 15101−15118.
(43) Pagliai, M.; Muniz-Miranda, F.; Cardini, G.; Righini, R.;
Schettino, V. Hydrogen bond dynamics of methyl acetate in
methanol. J. Phys. Chem. Lett. 2010, 1, 2951−2955.
(44) Pagliai, M.; Muniz-Miranda, F.; Cardini, G.; Righini, R.;
Schettino, V. Spectroscopic properties with a combined approach of
ab initio molecular dynamics and wavelet analysis. J. Mol. Struct. 2011,
993, 438−442.
(45) Muniz-Miranda, F. Modelling of Spectroscopic and Structural
Properties Using Molecular Dynamics; Firenze University Press (FUP):
Italy, 2014 .
(46) Liu, Y.; Lu, Z.; Zhu, H.; Hasi, W. Characterization of a
Chloride-Activated Surface Complex and Corresponding Enhancement Mechanism by SERS Saturation Effect. J. Phys. Chem. C 2017,
121, 950−957.
(47) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; et al. Gaussian 09,
revision D.01; Gaussian, Inc.: Wallingford, 2009.
(48) Becke, A. D. Density-functional thermochemistry. III. The role
of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.
(49) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti correlation-energy formula into a functional of the electron
density. Phys. Rev. B 1988, 37, 785−789.
(50) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence,
triple zeta valence and quadruple zeta valence quality for H to Rn:
Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7,
3297−3305.
(51) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate spin-dependent
electron liquid correlation energies for local spin density calculations:
a critical analysis. Can. J. Phys. 1980, 58, 1200−1211.
(52) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
Ab Initio Calculation of Vibrational Absorption and Circular
Dichroism Spectra Using Density Functional Force Fields. J. Phys.
Chem. 1994, 98, 11623−11627.
(53) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for
molecular calculations. Potentials for the transition metal atoms Sc to
Hg. J. Chem. Phys. 1985, 82, 270−283.
(54) Wadt, W. R.; Hay, P. J. Ab initio effective core potentials for
molecular calculations. Potentials for main group elements Na to Bi. J.
Chem. Phys. 1985, 82, 284−298.
(55) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for
molecular calculations. Potentials for K to Au including the outermost
core orbitals. J. Chem. Phys. 1985, 82, 299−310.
(56) Pagliai, M.; Muniz-Miranda, F.; Schettino, V.; Muniz-Miranda,
M. Competitive Solvation and Chemisorption in Silver Colloidal
Suspensions. Prog. Colloid Polym. Sci. 2011, 139, 39−44.
(57) Muniz-Miranda, M.; Pergolese, B.; Muniz-Miranda, F.;
Caporali, S. SERS effect from Pd surfaces coated with thin films of
Ag colloidal nanoparticles. J. Alloys Compd. 2014, 615, S357−S360.
(58) Gellini, C.; Deepak, F. L.; Muniz-Miranda, M.; Caporali, S.;
Muniz-Miranda, F.; Pedone, A.; Innocenti, C.; Sangregorio, C.
Magneto-Plasmonic Colloidal Nanoparticles Obtained by Laser
Ablation of Nickel and Silver Targets in Water. J. Phys. Chem. C
2017, 121, 3597−3606.
(59) Keresztury, G. In Handbook of Vibrational Spectroscopy;
Chalmers, J. M., Griffiths, P. R., Eds.; Wiley & Sons: Chichester,
UK, 2002; vol. 1; pp. 71−87.
(60) Krishnakumar, V.; Keresztury, G.; Sundius, T.; Seshadri, S.
Density functional theory study of vibrational spectra and assignment
of fundamental vibrational modes of 1-methyl-4-piperidone. Spectrochim. Acta, Part A 2007, 68, 845−850.
13537
DOI: 10.1021/acsomega.8b02174
ACS Omega 2018, 3, 13530−13537