Topics Covered
Background
Single-Walled Carbon Nanotubes (SWCNTs)
and Quantum Dots - Properties, Fluorescence and Applications
Sonicating Single-Walled Carbon Nanotubes
(SWCNTs) in Sodium Dodecyl Sulfate - a Description of the Experimental Process
Software Tools Used to Analyze the Data
from the Excitation/Emission Matrix-Scan
How the Nanosizer Software Program
Works
The Results that Emerged from this
Experiment
Conclusions and List of Tools Used
in this Experiment
Background
Single-walled carbon nanotubes
(SWCNs) and quantum dots have received much attention recently. These
nanomaterials fluoresce in the visible and IR regions; this fluorescence
can be used to characterize their properties and structure. The NanoLog™,
a modular spectrofluorometer from Horiba Scientific specifically designed for researching nanomaterials, is shown
to be able to rapidly (seconds to minutes) collect and analyze the instrument-corrected
fluorescence spectra of nanomaterials for characterization. Both SWCNs
in aqueous sodium dodecyl sulfate, and quantum dots have been studied
using the NanoLog™, which includes InGaAs near-IR detectors, CCD arrays,
or IR-sensitive photomultiplier tubes, and software for spectral analysis.
Single-Walled Carbon Nanotubes (SWCNTs) and Quantum Dots – Properties,
Fluorescence and Applications
Single-walled carbon nanotubes (SWCNs) and quantum dots, as well as related
nanomaterials, are under intense study because of their novel properties and
potential uses in the fields of materials science, biotechnology, and medicine.
Fluorescence of SWCNTs and quantum dots varies according to their size and shape.
Such fluorescence in the IR can be used to characterize the properties and structure
of these nanomaterials. Rapid spectral acquisition and analysis of nanomaterials
is useful in the fields of chemistry, biology, and materials science; therefore
Horiba Scientific
has designed a spectrofluorometer, the NanoLog™ (see
figure 1), specifically for such uses.
Figure 1. NanoLog™ spectrofluorometer from Horiba Scientific,
specifically designed to detect fluorescence from nanomaterials.
Sonicating
Single-Walled Carbon Nanotubes (SWCNTs) in Sodium Dodecyl Sulfate -
a Description of the Experimental Process
An uncharacterized mixture of SWCNs was sonicated for 30 min in a sodium dodecyl
sulfate solution in D2O at room temperature. The sample was placed
in a cuvette (path length = 5 mm) in a NanoLog™
spectrofluorometer set up with emission detection at right-angles to the excitation.
Excitation of the sample was performed with a 450 W Xe CW lamp shining into
a double-grating excitation monochromator (Spex® 180DF, 1200 grooves/mm
blazed at 330 nm). Excitation band-pass was set to 14.7 nm, and the excitation
was scanned from 550 nm to 800 nm in 5 nm steps. The emission spectrometer was
a single-grating TRIAX 320 (150 grooves/mm blazed at 1200 nm). Bandpass was
set to 12.5 nm. The emission spectrum was captured using a liquid-nitrogen-cooled
Symphony® CCD InGaAs array (512 × 1 pixel; see Figure 2) from 836.044
nm to 1359.93 nm, with 20s integration per scan, and 50 scans recorded. A silicon
photodiode was used as a reference detector.
Figure 2. Symphony® CCD array attached to
the TRIAX 320 spectrometer on the NanoLog™.
Software Tools
Used to Analyze the Data from the Excitation/Emission Matrix-Scan
After an excitation/emission matrix-scan is recorded, the data may be analyzed
with Horiba Scientific’s
Nanosizer™ software (patent pending), to assign spectral peaks to particular
SWCN structures. A sample screenshot of the Nanosizer™ software is presented
in figure 3. An overview of the Nanosizer™ algorithm is given in the paragraph
immediately below figure 3.
Figure 3. Screenshot of Nanosizer™ software used to assign
spectral peaks to SWCN structures.
How the Nanosizer
Software Program Works
The software selects a region of interest
within the matrix scan, and calculates first- and second-derivatives
of all excitation and emission channels. It then finds peaks in those
derivative surfaces, and generates a table of hypothetical values for
spectral bands, including amplitude of peak, center of excitation and
emission bands and their concomitant standard deviations. These hypothetical
coordinates are tested against a known spectral library; positive matches
are used to generate an improved hypothetical table, while negative
matches are used unchanged within the improved hypothetical table. A
double-convolution model is used to define each spectral component,
via lineshape functions of excitation peak, standard deviation, and
amplitude, with the emission peak, standard deviation, amplitude. The
model and data are used to compute a goodness-of-fit parameter (reduced
X2 or sum of squared residual errors).
If the residual-error sum is acceptable, the parameters are used for
a final assignment. If the residual-error sum is unacceptable, then
peaks may be added or deleted for re-parameterization.
The Results that
Emerged from this Experiment
Corrected spectra (signal/reference) of the SWCN mixture are presented as an
excitation-emission matrix in figure 4. To show the operation of the Nanosizer™
peak-characterization software, a simulation of corrected (signal/reference)
data was created based on known assignments and analyzed. In figures 5 and 6,
plot (figure 5) shows the assignment of spectral peaks to various SWCN structures
with (figure 6) a table of results. Included in the table of results is the
assigned radial breathing mode ωRBM of each species, which
can be used to calibrate the instrument, or compare to an independent Raman
measurement.
Figure 4. Corrected spectra (signal/reference) plotted
as a function of excitation and emission wavelength from SWCNs.
Figure 5. Assignment of spectral peaks by the Nanosizer™
software. Chirality is given as (n,m).
Figure 6. Table generated by the Nanosizer™ software,
based on analyzing the excitation-emission matrix. Columns from left to right
are: SWCN peak number, peak intensity, excitation λ (nm), peak emission
λ (nm), chirality (n,m), radial breathing mode ωRBM
(cm–1), and nanotube diameter dt (nm).
Conclusions and
List of Tools Used in this Experiment
The
NanoLog™ uses
state-of-the-art multi-channel near-IR wavelength-detection for rapid
and robust acquisition of photoluminescence excitation-emission matrices.
These matrices play a central role in the analysis of the diameter and
chirality of semiconductive species of SWCN mixtures. The Nanosizer™
software package incorporates a novel “double-convolution integral”
method (patent-pending) for fast and accurate analytical simulation
of photoluminescence excitation-emission matrices. The Nanosizer™ algorithm
is signified by its capacity to reduce the number of model parameters
by up to three orders of magnitude compared to conventional two-dimensional
(intensity vs. wavelength) multi-peak simulators. The Nanosizer™ generates
the complete excitation-emission matrix, yielding analytical solutions
for the chirality, diameter, and (n,m) values for all detected SWCNs
in a given sample.
Note: A complete set of references can be found by referring
to the original document.
Source: "Enhanced Characterization and Analysis of Nanomaterials
Using the Nanolog", Application Note by Horiba Scientific.
For more information on this source please visit Horiba
Scientific.