Seven compounds were isolated and characterized from the culture broth of the marine bacteria Nocardiopsis sp. (strain G057), which was isolated from sediment collecting at Cô Tô – Quảng Ninh. Their structures were determined by spectroscopic analysis inc

Cite this paper: Vietnam J. Chem., 2021, 59(2), 159-166 Article 
DOI: 10.1002/vjch.202000137 
159 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH 
SERS chemical enhancement by copper - nanostructures: Theoretical 
study of Thiram pesticide adsorbed on Cu20 cluster 
Truong Dinh Hieu
1,2
, Ngo Thi Chinh
1,2
, Nguyen Thi Ai Nhung
3
, Duong Tuan Quang
4
, 
Dao Duy Quang
1,5*
1
Institute of Research and Development, Duy Tan University, Da Nang, 50000, Viet Nam 
2
Faculty of Natural Sciences, Duy Tan University, Da Nang, 50000, Viet Nam 
3
Department of Chemistry, University of Sciences, Hue University, Hue City, Thua Thien Hue 49000, Viet Nam 
4
University of Education, Hue University, Hue City, Thua Thien Hue 49000, Viet Nam 
5
Department of Environmental and Chemical Engineering, Duy Tan University, Da Nang, 50000, Viet Nam 
Submitted August 7, 2020; Accepted November 9, 2020 
Abstract 
Surface-enhanced Raman spectroscopy (SERS), a surface-sensitive technique, allows the practicability of detecting 
chemical compounds in ultra-low concentration. In this work, a chemical enhancement mechanism of SER process of 
Thiram pesticide adsorbed on copper nanomaterial surface was proposed based on density functional theory (DFT) 
approaches. Structural and electronic properties of Thiram and Thiram-Cu20 complexes were optimized using PBE 
method with LanL2DZ basis set for copper atoms and cc-pVDZ basis set for the non-metal atoms. In the most stable 
adsorption configuration, Thiram interacted with Cu20 cluster via two S(sp
2
) atoms. The main peaks on normal Raman 
spectrum of Thiram were characterized at 371, 576, 1414 and 1456 cm
-1
 responsible for the stretching vibrations of 
C–S, C=S, S–C–S and C–N groups, respectively. Otherwise, the main peaks of Thiram-Cu20 SERS spectrum were 
found at 534, 874, 982, 1398 and 1526 cm
-1
 corresponding to the stretching vibrations of S–S, C-S, S–C–S, C–N and 
CH3–N bonds, respectively. The SERS chemical enhancement of Thiram by Cu20 cluster was about 2 and 6 times 
stronger than those obtained from Ag20 and Au20 cluster, respectively. The chemical enhancement mechanism was also 
explained by analyzing HOMO and LUMO energies gap and density of states. 
Keywords. Thiram, copper cluster, Raman, SERS, DFT. 
1. INTRODUCTION 
Pesticides are chemical compounds used in modern 
agriculture to kill insects, fungus, bacteria, weed and 
rodents. They are respectively named as insecticides, 
fungicides, bactericides, herbicides and rodenticides. 
By the structure, pesticides can be divided into 
organochlorines, organophosphates, carbamates and 
triazines.
[1,2]
 An increasing utilization of pesticides 
in agriculture results in several severe problems on 
environment and human health. 
Thiram (tetramethyl-thiuram disulfide or 
bis(dimethyl-thiocarbamoyl) disulfide) (C6H12N2S4) 
is a carbamate-categorized pesticide. Its molecular 
structure has two dimethyl-dithio-carbamate groups 
– (CH3)2N–CS2 linked together by a disulfide bridge 
(S–S). Thiram has been used in many countries as 
fungicide to protect fruits, vegetables, ornamental 
and turf crops from a variety of fungal diseases.
[3-6]
This compound is also used to protect fruit trees and 
ornamental fruits from damage of rabbit, rodent and 
deer.
[7]
For many decades, surface-enhanced Raman 
spectroscopy (SERS) has intensively been 
investigated for its electromagnetic field 
enhancement near the nano-scale metallic surfaces 
of coinage metals (i.e. gold, silver and copper). 
Despite of intensive research attempts SERS 
chemical enhancement mechanism is still unclear 
mainly due to the relatively complicated enhancing 
factors and inconsistent experimental results. The 
advantages of SERS are that it magnifies Raman 
signals corresponding to the adsorbed compounds 
from 10
6
 to 10
10
 times. Therefore, SERS technique 
has increasingly been utilized to improve detection 
of chemicals at trace concentrations. Attracted by its 
great advantages, many researches have employed 
SERS to analyze different chemical pesticides, 
Vietnam Journal of Chemistry Dao Duy Quang et al. 
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 160 
including Thiram, accumulated either in the 
environment or in agricultural products. 
Kang et al. analyzed the SERS spectrum of 
Thiram adsorbed on silver surface.
[8]
 Their results 
revealed that the peaks of Thiram located in the 
region below 1000 cm
-1
 (related to C–S, C–S–S 
assignments) are decreased or even disappeared in 
the SERS spectrum; whereas, others characterized 
for C–N and CH3NC are enhanced, especially C–N 
stretching mode at 1372 cm
-1
. These phenomena 
were also confirmed by Verma et al. using silver 
nanodendrites.
[9]
The prediction of Raman and SERS spectra has 
commonly been investigated using density 
functional theory (DFT). Metallic cluster models are 
often used to reproduce nanoparticle surface. The 
complexes produced from interaction between an 
analyzed ligand and a metallic cluster can be utilized 
to predict their SERS spectra. Rajalakshmi et al. 
determined geometrical and electronic structures of 
2-propylpiridine-4-carbothioamide as well as studied 
infrared, Raman spectra.
[10]
 In their work, various 
DFT functionals including PBEPBE, SVWN, 
HCTH, B3LYP, mPW1PW91, B3PW91 combined 
with aug-cc-pVDZ basis set were chosen as 
computational strategies for spectra prediction. The 
research indicated that the B3LYP/aug-cc-pVDZ 
model results in the lowest deviations in the 
prediction of structure and vibrational spectra. 
Recently, An et al. investigated surface-enhanced 
Raman scattering of melamine (C3H6N6) on silver 
substrate using experimental and DFT studies with 
the B3LYP/6-31G(d) method.
[11]
 Silver cluster 
models includin ... hemistry SERS chemical enhancement by copper - nanostructures: 
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 163 
1012 cm
-1
, 1414 cm
-1
 and 1531 cm
-1
 (table 1). The 
Raman intensities of these peaks see a respective 
rise of 122, 102, 50 and 175 times in the SERS 
spectrum of Thiram-Cu20 complex. Otherwise, the 
indices for Raman enhancement vary from 21 to 104 
times for Thiram-Ag20 complex and from 12 to 54 
times for Thiram-Au20 complex. The enhancement is 
mainly due to the stretching vibrations of CH3N, CN 
groups and the wagging vibrations of CH3, 
CH3NCH3 groups. However, some other peaks only 
witness a marginal-to-non enhanced intensity, such 
as those at 371, 576, 1430 and 1456 cm
-1
 (figure 
4A). In particular, two peaks at 371 and 1430 cm
-1
are both disappeared in the SERS spectra obtained 
from all three metal complexes. These peaks relate 
to the stretching vibration of C–S bond and the 
scissoring bending vibrations of CH3, CH3NC 
groups (table 1). 
In addition, the Raman intensity of highest peak 
in SERS spectrum of Thiram-Cu20 at 982 cm
-1
 is 2 
times higher than that of Thiram-Ag20 and 6 times 
higher than that of Thiram-Au20. The other 
noticeable peaks of Thiram-Cu20 complex (i.e. 534, 
1398 and 1526 cm
-1
) also have higher Raman 
activities than the ones of other complexes. Overall, 
Raman figures obtained for Thiram-Cu20 are from 
1.2 to 2.4 times higher than those of Thiram-Ag20 
and from 3.3 to 4.2 times higher than those of 
Thiram-Au20. 
Figure 4: (A) Raman spectrum of Thiram and SERS 
spectra of the most stable complexes: (B) Thiram-
Cu20, (C) Thiram-Ag20 and (D) Thiram-Au20 
Table 1: Vibrational assignments of normal Raman spectrum of Thiram and SERS spectra of Thiram 
adsorbed on Cu20, Ag20 and Au20 clusters 
Raman SERS-Cu20 SERS-Ag20 SERS-Au20 Assignments 
301 (6.0) 279 (80.2) 236 (47.2) 210 (60.0) ρ(CH3), σ(NCS), σ(CSS) 
371 (14.3) – – – υ(CS), σ(CH3NC) 
451 (2.5) 445 (76.6) 396 (39.9) 401 (20.1) σ(CS), σ(NCS), σ(CH3NC), 
– 490 (171.3) 525 (129.1) – ω(SCS), ω(CH3NCH3) 
553 (3.6) 534 (439.9) 540 (356.1) 538 (122.3) υ(SS), ω(SCS), ω(CH3NCH3) 
576 (24.9) 571 (109.9) 559 (206.1) 557 (66.3) σ(CH3NCH3), υs(SCS), υas(CSS) 
873 (3.3) 874 (238.8) 871 (105.2) 870 (38.4) υs(CH3NCH3), υs(CS) 
1012 (18.9) 982 (1924.5) 991 (984.2) 983 (339.7) υas(SCS), ω(CH3), υ(CH3N), σ(CH3NC) 
1102 (4.3) 1126 (103.9) 1092 (12.4) 1090 (8.3) ρ(CH3), ω(CH3) 
1178 (1.8) 1161 (84.0) 1159 (23.3) 1155 (26.5) ω(CH3), ρ(CH3), υas(SC=S) 
1297 (1.9) 1272 (112.6) 1274 (58.8) 1267 (12.9) υas(CH3NCH3), ω(CH3), υas(SCS) 
1414 (20.7) 1398 (1041.7) 1398 (440.6) 1399 (248) υ(CN), ω(CH3) 
1430 (14.4) – – – σ(CH3) 
1456 (50.4) 1455(157.6) 1457 (53.7) 1458 (46.8) σ(CH3) 
1531 (3.6) 1526 (630.4) 1549 (375.8) 1554 (193.3) υ(CN), ω(CH3), σ(CH3) 
Values in parentheses are calculated Raman activities; (υ) = stretching (with υs = symmetric stretching and 
υas = anti-symmetric stretching), σ = scissoring bending, ρ = rocking, ω = wagging, τ = twisting. 
3.4. Chemical enhancement mechanism 
It has been widely accepted that the SERS 
phenomenon generally stems from electromagnetic 
and chemical enhancement mechanisms. The former 
is based on the amplification of the light by the 
excitation of localized surface plasmonic resonances 
(LSPRs). The latter primarily refers to charge 
transfer (CT) process, where the excitation 
wavelength resonates with the metal molecule 
Vietnam Journal of Chemistry Dao Duy Quang et al. 
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 164 
charge transfer electronic states.
[17]
 The chemical 
enhancement mechanism of Thiram adsorbed on 
Au20, Ag20 and Cu20 clusters is illustrated in figure 5 
and table 2. 
In particular, figure 5 summarizes total density 
of states (DOS) spectrum of Thiram, Au20, Ag20 and 
Cu20 bare clusters in comparison with the ones of 
their complexes. The highest occupied molecular 
orbital (HOMO) and the lowest unoccupied 
molecular orbital (LUMO) distributions of the 
studied structures are also displayed with their 
corresponding energy values (EH and EL). The 
extended values of HOMO-LUMO energy gap ( E) 
are also included. Furthermore, partial density of 
states (PDOS) analyses provides contributing 
proportion of Thiram and its coordinated metal 
clusters in the complexes. LUMO and HOMO are 
also indicated in order to analyze the CT tendency of 
electron densities. 
Firstly, the difference between HOMO energy 
and LUMO one, i.e. HOMO-LUMO gap or E, is a 
good indicator to evaluate kinetic stability and 
chemical stability. Regarding table 2, the E values 
of the bare clusters accord with the order: Au20 > 
Ag20 > Cu20 with their corresponding figures 1.89, 
1.67 and 1.46 eV, respectively. The narrowest 
energy gap of Cu20 (1.46 eV) indicates its highest 
reactivity towards Thiram in comparison with Au20 
and Ag20 clusters. Expectedly, the HOMO-LUMO 
energy gaps of the complexes are also in the similar 
order: Thiram-Au20 (1.41 eV) > Thiram-Ag20 (1.28 
eV) > Thiram-Cu20 (0.95 eV) which shows a reverse 
order of the stability. 
Thus, the narrower energy gaps of the Cu20 bare 
clusters and of the Thiram-Cu20 complex are more 
conducive transfer of electron densities from the 
ligand to the cluster than those carried out by Ag20 
and Au20 clusters. The easier electronic transfer also 
explains for the most marked enhancement by SERS 
for Thiram adsorbed on Cu20 (figure 4 and table 1).
Figure 5: Density of states (DOS) spectrum of Thiram, Au20, Ag20 and Cu20 bare clusters and their 
complexes with Thiram (Thir-Au20, Thir-Ag20, Thir-Cu20). LUMO and HOMO distributions are presented on 
the right and left hand sides of each graphic. The LUMO and HOMO energies are indicated besides the 
vertical dotted lines with their HOMO-LUMO gap ( E) in eV unit. The percentage values correspond to  
contribution of Thiram and the metal clusters to LUMO and HOMO orbitals 
Vietnam Journal of Chemistry SERS chemical enhancement by copper - nanostructures: 
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 165 
Secondly, information on the frontier orbitals of 
the formed complexes clarifies the interaction 
mechanism between the pesticide molecule and the 
clusters. And the most important interactions are 
between HOMO and LUMO of Thiram and those of 
the clusters. Based on the energy gap, the donation or 
back-donation of electrons can be revealed. Based on 
the calculated data in table 2, the energy differences 
between LUMO of Thiram and HOMO of the clusters 
are also ranged in declined order: Au20 > Ag20 > Cu20 
with the respective values being 4.31, 3.27 and 3.16 
eV. In contrast, the energy gaps between LUMO of 
the Au20, Ag20 and Cu20 clusters and HOMO of 
Thiram register are similar order but with 
considerably smaller values i.e. 0.77, 1.59 and 1.49 
eV, respectively. These imply that the HOMO-
LUMO energy gaps of the forward donation 
(Thiram M20, with M represents the metal cluster) 
are larger than the ones represented for backward 
donations (M20 Thiram). Hence, Thiram is adsorbed 
on the metal cluster by donating its electron densities 
to the cluster. This electron-transfer tendency from 
organic molecules to metallic cluster is in agreement 
with the previous studies.
[18-20]
Table 2: HOMO and LUMO energies and HOMO-
LUMO energy gap 
HOMO LUMO E (eV) 
Thir -4.72 -1.53 3.19 
Au20 -5.84 -3.95 1.89 
Ag20 -4.80 -3.13 1.67 
Cu20 -4.69 -3.23 1.46 
Thir-Au20 -4.94 -3.53 1.41 
Thir-Ag20 -4.26 -2.98 1.28 
Thir-Cu20 -3.94 -2.99 0.95 
This observation is further confirmed by 
analyzing partial density of states (PDOS) (figure 5). 
The contribution percentages of Thiram and the 
clusters to LUMO and HOMO indicate that electron 
densities are always transferred from Thiram to the 
cluster during the transition from LUMO to HOMO 
of the complexes. Regarding Thiram-Cu20, 75  of 
LUMO electron density is localized on Thiram while 
only 25  is found on the Cu20 cluster. However, 
only 9  of HOMO electron density localizes on 
Thiram, the corresponding figure for the cluster Cu20 
is 91 %. This means that 64  electron densities are 
transferred from Thiram to the Cu20 cluster. 
Consistent phenomena are observed in regard to 
Au20 and Ag20 clusters with the transfer of 16  and 
73  electron densities, respectively. 
4. CONCLUSIONS 
Structural, electronic and spectroscopic properties of 
Thiram and its complexes with Cu20, Ag20 and Au20 
are computationally investigated using DFT method. 
Normal Raman spectra of Thiram and SERS 
spectrum of its three complexes are projected. The 
results show that: 
+ Thiram contains two co-planar C2NCS2 groups 
and the interactive sites of Thiram are mainly found 
at the its sulfur atoms (especially at S(sp
2
) atoms, i.e. 
S3 and S4). 
+ Thiram interacts with Cu20 cluster via two or 
more sulfur atoms. The stability of Thiram-Cu20 
complexes depends on the number of interaction 
between Cu20 cluster and S(sp
2
) atom. The more 
S(sp
2
) atom interact with the Cu20 cluster, the more 
stable the complex is. 
+ Normal Raman spectrum of Thiram shows 
several main peaks including the stretching vibration 
of C–S bond and scissoring bending vibration of 
CH3 groups. Otherwise, the main peaks of SERS 
spectrum of Thiram-Cu20, Thiram-Ag20 and Thiram-
Au20 complexes relate to N atom and the wagging 
vibration of CH3 groups. 
+ The SERS chemical enhancement for Thiram 
derived by Cu20 cluster is 2 and 6 times higher than 
those attained by Ag20 and Au20 clusters. 
+ The most enhanced SERS signals of Thiram 
adsorbed on Cu20 cluster are firstly related to its 
lowest HOMO-LUMO energy gap by referencing to 
the Au20 and Ag20 clusters. Moreover, during the 
adsorption, the charge transfer prevails through the 
forward donation direction from Thiram to the metal 
clusters (Thiram M20). The energy gap between 
LUMO of Thiram and HOMO of Cu20 is the lowest 
compared with those of Au20 and Ag20 cluster. The 
highest charge transfer from Thiram to cluster is also 
obtained for the copper one. And this tends to the 
highest SERS signals obtained when Thiram is 
adsorbed on the Cu20 cluster. 
The predicted results suggest a magnification-
enhanced and cost-effective copper-based 
nanomaterial as a potential alternative for expensive 
inert metals, such as silver or gold, in SERS 
applications. The most noticeable downside is its 
sensitivity to ambient oxidization. The disadvantage 
is less pronounced if the material is expected for 
portable or one-use purposes. 
Acknowledgments. This research is funded by 
Vietnam National Foundation for Science and 
Technology Development (NAFOSTED) under grant 
number 103.03-2018.366. 
Vietnam Journal of Chemistry Dao Duy Quang et al. 
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 166 
Conflict of interest. The authors declare no conflict 
of interest. 
REFERENCES 
1. R. Jayaraj, P. Megha, P. Sreedev. Organochlorine 
pesticides, their toxic effects on living organisms and 
their fate in the environment, Interdiscip. Toxicol., 
2016, 9, 90-100. 
2. R. C. Gupta. Carbamate Pesticides, in: Encycl. 
Toxicol., Elsevier, 2014, 661-664. 
3. V. K. Sharma, J. S. Aulakh, A. K. Malik, Thiram: 
degradation, applications and analytical methods, J. 
Environ. Monit., 2003, 5, 717-723. 
4. B. Gupta, M. Rani, R. Kumar. Degradation of thiram 
in water, soil and plants: A study by high-
performance liquid chromatography, Biomed. 
Chromatogr., 2012, 26, 69-75. 
5. H. Dies, M. Siampani, C. Escobedo, A. Docoslis. 
Direct detection of toxic contaminants in minimally 
processed food products using dendritic surface-
enhanced Raman scattering substrates, Sensors, 2018, 
18, 1-11. 
6. P. Stathi, K. C. Christoforidis, A. Tsipis, D. G. Hela, 
Y. Deligiannakis. Effects of dissolved carboxylates 
and carbonates on the adsorption properties of 
thiuram disulfate pesticides, Environ. Sci. Technol., 
2006, 40, 221-227. 
7. G. M. Linz, M. L. Avery, R. A. Dolbeer, eds. 
Ecology and Management of Blackbirds (Icteridae) 
in North America, CRC Press, Boca Raton: CRC 
Press, 2017. 
8. J. S. Kang, S. Y. Hwang, C. J. Lee, M. S. Lee. SERS 
of dithiocarbamate pesticides adsorbed on silver 
surface; Thiram, Bull. Korean Chem. Soc., 2002, 23, 
1604-1610. 
9. A. K. Verma, R. K. Soni. Silver nanodendrites for 
ultralow detection of thiram based on surface-
enhanced Raman spectroscopy, Nanotechnology, 
2019, 30, 385502-385516. 
10. K. Rajalakshmi, M. Vetrivel. Optimization and 
vibrational study of 2-propylpyridine-4-
carbothioamide by DFT - A theoretical study, Int. J. 
ChemTech Res., 2016, 9, 306-316. 
11. N. T. T. An, D. Q. Dao, P. C. Nam, B. T. Huy, H. N. 
Tran. Surface enhanced Raman scattering of 
melamine on silver substrate: An experimental and 
DFT study, Spectrochim. Acta Part A Mol. Biomol. 
Spectrosc., 2016, 169, 230-237. 
12. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. 
Scuseria, M. A. Robb et al., Gaussian 16 Rev. A.03, 
2016. 
13. J. P. Perdew, K. Burke, M. Ernzerhof. Errata: 
Generalized gradient approximation made simple, 
Phys. Rev. Lett., 1997, 78, 1396. 
14. M. K. Kesharwani, B. Brauer, J. M. L. Martin. 
Frequency and zero-point vibrational energy scale 
factors for double-hybrid density functionals (and 
other selected methods): Can anharmonic force fields 
be avoided?, J. Phys. Chem. A., 2015, 119, 1701-
1714.
Corresponding author: Dao Duy Quang 
Institute of Research and Development, Duy Tan University 
3, Quang Trung, Da Nang, 50000, Viet Nam 
E-mail: daoduyquang@duytan.edu.vn.