The absorption properties of gold nano conjugated with proteins
HNUE JOURNAL OF SCIENCE
DOI: 10.18173/2354-1059.2020-0045
Natural Sciences 2020, Volume 65, Issue 10, pp. 29-35
THE ABSORPTION PROPERTIES OF GOLD NANO CONJUGATED
WITH PROTEINS
Luong Thi Theu1, Le Anh Thi2, Tran Quang Huy1, Nguyen Quang Hoc3
and Nguyen Minh Hoa4
1Faculty of Physics, Hanoi Pedagogical University 2
2Institute of Research and Development, Duy Tan University
2Faculty of Natural Sciences, Duy Tan University
3Faculty of Physics, Hanoi National University of Education
4Faculty of Basic Sciences, Hue University of Medicine and Pharmacy
Abstract. The absorption properties of protein-conjugated metallic nanoparticles are
theoretically investigated based on the Mie theory and the core-shell model. Our
numerical calculations show that this finding is in good agreement with previous
experiments. We provide a better interpretation of the origin of optical peaks in the
absorption spectrum of the nanoparticle complex system. Our results can be used in
biomedical applications.
Keywords: gold nanoparticle, BSA protein, Mie theory.
1. Introduction
Gold nanoparticles (GNPs), with a diameter between 1 nm and 100 nm, have been
widely used in chemical and biological sensors because of their excellent physical and
chemical properties. The unique optical property of GNPs is one of the reasons that GNPs
attract immense benefits from various fields of science, especially in the development of
sensors. The spherical GNP solutions show a range of vibrant colors including red, blue,
and violet when the particle size increases, and they can be used to dye glass in ancient
times. The strong color is caused by the strong absorption and scattering of 520 nm light [1],
which is the result of the collective oscillation of conduction electrons on the surface of
GNPs when they are excited by the incident light. This phenomenon is called surface
plasmon resonance (SPR), and it depends greatly on particle size and shape. Therefore,
the SPR peak can be adjusted by manipulating the size of GNPs, and this property cannot
be observed on bulk gold and GNPs with a diameter smaller than 2 nm.
The SPR peak is not only sensitive to the size and the shape, but also many factors
such as a protective ligand, refractive index of solvent, and temperature. The distance
Received June 17, 2020. Revised October 16, 2020. Accepted October 23, 2020.
Contact Nguyen Minh Hoa, e-mail address: nguyenminhhoa@hueuni.edu.vn
29
Luong Thi Theu, Le Anh Thi, Tran Quang Huy, Nguyen Quang Hoc and Nguyen Minh Hoa
between particles particularly shows the great influence on SPR. Thus, the red-shifting
and the broadening of the peak are observed when GNPs are synthesized due to analyte
binding. The color change of synthesized GNPs from red to blue is the principle of
colorimetric sensors. Several recent pieces of research and reviews provide a detailed
discussion of the factors that affect the SPR of GNPs [2-6].
Bovine serum albumin (BSA) protein has been widely used in the field of biophysics
and medical science, due to its low cost, structural/ functional similarity to human serum
albumin (HSA) [7]. A recent study found that the ribosylation of BSA resulted in reactive
oxygen species (ROS) accumulation which killed breast cancer cells [8]. Particularly the
anomalous thermal denaturing of proteins increased signal in the tests, biochemical
reactions [9, 10]. This effect is strong in BSA proteins and is particularly useful for the
design of bio-sensors and devices.
In recent years, the plasmonic properties of metallic nanoparticles are of great interest
because they have various potentially technological applications, especially the magnetic
nanoparticles (NPs). Localized surface plasmon resonances with gold nanoparticles have
many applications for a variety of application areas e.g. chemical analysis and catalytic,
detect biomolecules, pharmaceutical, diagnosis, imaging, and therapy [1, 11, 12].
Complex systems of biological gold nanoparticles have also been investigated to
construct functional devices for cell imaging, drug delivery, and biomolecule detection.
Bovine Serum Albumin (BSA) proteins have been particularly useful in this issue [1].
The BSA substances not only prevent AuNPs from together combination but also are
effective for treatment delivery and attaching AuNPs in living matter. Because of their
large scattering crossing sections, BSA-AuNPs themselves can be imaged under white
light illumination. Moreover, adjusting the optical plasmon resonance on the visible
spectrum is implemented by changing the particle size and shape that have been especially
helpful in optimizing the application of complex systems of biological gold nanoparticles.
In this paper, we theoretically study the optical properties of AuBSA core-shell nano
using the Mie theory and effective medium approximation, which has been synthesized
experimentally in Ref. 13 in the visible range.
2. Content
2.1. Theoretical background
Calculating exactly the number of BSA molecules on a gold nanoparticle’s surface
based on the absorption spectrum and the extended Mie theory [13] shows that the core-
shell model and the effective medium approximation provides a good agreement between
theoretical calculations and experimental for spherical nanoparticles. Now, we apply
these theories to the complex system to investigate and predict the properties of protein-
conjugated gold nanoparticles. An idea of modeling nanoparticles conjugated
nanoparticles as a core-shell structure has been widely used [14, 15]. In this work, the
absorption and scattering of AuNP conjugated BSA protein in aqueous solutions are
theoretically considered. The system is formed when BSA and AuNP proteins are placed
in water. Some of the water is mixed with protein and this aqueous solution of BSA is
attracted to AuNP through Van der Waals interaction. As a result, a protein conjugated
nanoparticle is formed in water as shown in Figure 1.
30
The absorption properties of gold nano conjugated with proteins
Protein
NP
NP
r2
r1
r1
Figure 1. The core-shell model for protein-conjugated gold nano
The general solution to the problem of scattering of a spherical metal sphere
according to electrodynamics theory was first proposed by Mie in 1908 [16]. Mie's theory
applied an overview theory of scattering on small particles to explain the color changing
of the colloidal gold nanoparticles with arbitrary size and shows s good agreement with
experimental results. When the radius of the nanoparticles is much smaller than the
wavelength of the incident light (
, or an approximation d max /10), the Mie
d
coefficients can be simplified by quasi-static approximations. Thus, using the exact
solution of Mie theory is necessary to calculate accurately the extinction, scattering, and
absorption coefficients cross-section of isotropically coated spherical nanoparticles are
given by [17].
2
C
=
(2n +1)Re a +b ,
n n
(
)
ext
n=1
2
2
(1)
(2)
C
=
(2n +1) a 2 + b
,
(
)
scat
n
n
n=1
Cabs = C −C
.
ext
scat
where
mp (mka)'p (ka) −p (ka)'p (mka)
mp (mka)'p (ka) − p (ka)'p (mka)
p (mka)'p (ka) − mp (ka)'p (mka)
p (mka)'p (ka) − mp (ka)'p (mka)
an =
,
b =
,
n
C
i
Q =
,
i
R2
in which, Qi are the extinction, scattering, and absorption efficient, with i = [ext, scat, abs]
is running index and R gold nanoparticle radius.
is Riccati–Bessel function of the first
Ns
and second kind, and m =
, Ns and N are the refractive index of the noble metal sphere
N
31
Luong Thi Theu, Le Anh Thi, Tran Quang Huy, Nguyen Quang Hoc and Nguyen Minh Hoa
and the surrounding medium (perovskite), respectively, and
is the
k = 2 /
the dielectric function of core-shell spherical and n represent the
wavenumber with
mode. Expansion of infinite series exhibits the different excitation symmetry like a dipole,
quadrupole, and octupole corresponding to different values of n, respectively. Since it is
very difficult to find the solution of the sum of infinite series, we can easily handle the
situation if we truncated the series up to a certain value of n. If we are interested to study
dipolar effects, choose n = 1, for quadrupolar n = 2, and so on.
2.2. The absorption efficiency of BSA protein-conjugated gold nano
An effective dielectric function of core-shell nanoparticle dispersed in a solution can
be found from Maxwell-Garnett theory as
2
2
r
r
1
1
= 1+ 2
+ 2 1−
,
1
2
a
r
r
2
2
2
2
r
r
1
r
1
r
(3)
= 1−
+ 2 +
,
1
2
b
2
2
2a
b
=
,
in which,
,
1 and 2 are the wavelength of the incident in a vacuum, the dielectric
function for the core (Au), shell (BSA + surrounding medium), respectively. Parameters
and analytical expressions for these dielectric functions can be taken from a previous
study [18]. We introduce the filling factor of protein BSA on metallic surface f,
2 = f protein +(1− f )m , where
is the dielectric constant of the medium. and are
m
a b
n n
given by
R 1
−m
an =
(kR)2 ( −m ) +
,
2
i
4
2 8
1+ (kR)2 log kR
−
(kR)2 ( −m )
(
)
m
m
(4)
R −m
1
b =
2
+ (kR)2 ( −m ) .
n
2
+m
4
Figure 2 shows the absorption efficiency of BSA conjugated gold nanoparticles in
water. We found that Qabs behaves as a function of the wavelength. Here, we take that
r =10 nm for the AuNP and r =11.5 nm for the shell. The recent experiments indicate
1
2
that such a configuration corresponds to a BSA monolayer around the Au core [18]. There
is a very good agreement with the reported data in Ref. 18 for the AuNP/water system.
32
The absorption properties of gold nano conjugated with proteins
Figure 2. The absorption of AuNPs in water with BSA in the visible spectrum
and the diameter of AuNPs in the calculations is 20 nm
We assume that the equation is independent of frequency and a complex function
= −2
1 ( )
that depends on the energy. The resonant condition is satisfied when
and
m
2 is small or weakly dependent . The Eq.1 has been used to explain the absorption
spectrum of small metal nanoparticles both qualitatively and quantitatively. Using Mie
theory, we obtained the absorption coefficient at the maximum wavelength.
16Vm3/2
3
2 ()
Q =
,
abs
2
() + +22 ()
m
1
(5)
= + i
where
is the effective dielectric function of the object calculated by
1 ( )
2 ( )
Eq.4, is the imaginary part of
,
m is the dielectric constant of the medium,
2 ( ) ( )
4
V = R3 is the volume of one BSA protein molecule and R is the radius of the
3
nanoparticle complex. We also show two theories that have a good agreement that
() + = 0
maxima of the absorption spectrum of nanoparticle exhibit at
localized surface plasmon resonance of a spherical nanoparticle complex is at
() + = 0
. While the
m
.
m
3. Conclusions
In conclusion, we have presented a comprehensive explanation for optical peaks of
BSA-conjugated gold nanoparticles. The peak at the wavelength of 510 nm is due to
33
Luong Thi Theu, Le Anh Thi, Tran Quang Huy, Nguyen Quang Hoc and Nguyen Minh Hoa
biological molecules binding on nanoparticles and strongly depends on the dielectric
function of the protein and the adsorption of protein on gold nanoparticles. The results
show that there is a good agreement between theory and experiment. Our work shows
that the finite size of the nanoparticles may play an important role in the plasmon spectral
shift and it is directly relatedto the number of protein molecules attached to the AuNP surface.
Acknowledgment. This work was financially supported by the Hue University of Science
and Technology under grant number DHH2018-04-83.
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