Chrominance to Dimension: A Real-Time Method for Measuring the
Size of Single Gold Nanoparticles
Chao Jing,
†
Zhen Gu,
†
Yi-Lun Ying,
†
Da-Wei Li,
†
Lei Zhang,
†,‡
and Yi-Tao Long*
,†
†
Key Laboratory for Advanced Materials & Department of Chemistry, East China University of Science and Technology, 130 Meilong
Road, Shanghai 200237, P. R. China
‡
School of Materials Science & Engineering, Nanjing University of Posts and Telecommunications, 9 Wenyuan Road, Nanjing
210046, P. R. China
*
S
Supporting Information
ABSTRACT: Noble metal nanoparticles have excellent
optical and chemical properties and are widely used in optics,
sensors, and biomedicines. The inherent characteristics of
metal nanoparticles, particularly their size, play important roles
in their applications. The ability to readily measure the size of
single nanomaterials on-site is crucial to the rapid development of single-particle sensors. In this study, we developed a facile and
real-time method for estimating the diameter of single gold nanoparticles (GNPs) that range from 35 to 110 nm in diameter; this
technique uses the chrominance of the GNP’s plasmon resonance scattering light that is captured by a dark-field microscope
(DFM). The RGB (three primary colors, red, green, and blue) chrominance information from the dark-field image can be
directly converted into the diameters of the GNPs using the relationship between the particle size and the scattering light peak
wavelength; this conversion was carried out using Matlab program based on an RGB-To-Wavelength (RTW) process. This
approach is more convenient, less time-consuming, and enables observation under arbitrary conditions compared to the scanning
electron microscopy (SEM) technique. The differences between the diameters of the GNPs that were calculated using this
method and those that were measured using SEM were less than 5 nm. The RTW method has also been applied in the
monitoring of the refractive index of the media surrounding the GNPs, and their dynamic acting within cells in real-time.
L
ocalized surface plasmon resonance (LSPR) occurs in
plasmonic metallic particles (Au, Ag, Cu) which results
from the interaction between incident light and surf ace
electrons.
1
This phenomenon has attracted the attention of
numerous researchers.
1−5
Plasmonic nanoparticles have been
widely used in biology, chemistry, and photonics because of
their unique optical and chemical properties.
6− 9
Their
absorption and scattering properties can be utilized in
biomolecular binding, heavy metal ions detection, and
membrane transfer applications.
10−12
Because of their excellent
biocompatibility as well as their facile synthesis and
modification, the use of gold nanoparticles (GNPs) has
promoted advancements in the fields of biomedical detection,
drug delivery, and disease diagnosis.
13−15
For instance, protein-
labeled GNPs account for a large proportion of plasmonic-
based biosensors in cancer detection and biorecognition.
16
Notably, dark-field microscopy (DFM) allows plasmon
resonance scattering light to be used for intracellular detection
at the single-particle level.
17,18
GNPs have been used as contrast
agents in cell imaging and in vivo biosensing because of their
nontoxicity and lack of photobleaching or blinking.
2
Their
scattering intensity is also stronger than the fluorescence from
dyes and quantum dots.
19−21
According to previous studies, the
scattering light of a 50 nm GNP can exceed the fluorescence of
a chromophore or a quantum dot by a factor of over 1
million.
22
Another important application of plasmonic nano-
particles is photothermal therapy, which is based on the near-
infrared absorption of nanomaterials and can be tuned by
adjusting the size, shape and composition of the particles.
23,24
In biological sensing and detection applications, the size,
shape, composition, surrounding medium, and electron density
of the GNPs can impact their LSPR properties and sensing
efficiencies.
25,26
Especially, the particle size plays a crucial role
in its LSPR characteristics and biosensor applications.
27−29
Large nanoparticles (diameter d > 200 nm) exhibit strong
retardation effects, which invalidate the quasi-static approx-
imation that the plasmon resonance from the nanoparticles is
negligible. In contrast, the diameter of small nanoparticles
(diameter d < 20 nm) is less than the mean free path of the
moving electrons; therefore, the surface electrons do not
oscillate with enough energy to generate scattering light.
30,31
The sensitivity of nanoparticle-based cellular sensors is also size
dependent. GNPs that range from 2 to 100 nm in diameter
have been used to investigate the uptake process in living cells;
nanoparticles that have 40−50 nm diameters exhibited the
strongest effect.
32
In plasmon resonance energy transfer
(PRET) detection, the efficiency of the energy transfer is
relied on the overlap of the energy levels, which is determined
by the sizes of the particles.
33
In addition, when GNPs are used
as catalysts, smaller particles are more active than larger
Received: November 23, 2011
Accepted: April 14, 2012
Published: April 14, 2012
Article
pubs.acs.org/ac
© 2012 American Chemical Society 4284 dx.doi.org/10.1021/ac203118g | Anal. Chem. 2012, 84, 4284−4291