Supporting information
Optical rotation by plasmonic circular dichroism
of isolated gold nanorod aggregates
Kamalesh Chaudhari†,‡ & Thalappil Pradeep‡,*
†Department of Biotechnology and ‡DST Unit of Nanoscience (DST UNS) and
Thematic Unit of Excellence (TUE), Department of Chemistry, Indian Institute of
Technology Madras, Chennai 600 036, India.
*Address correspondence to pradeep@iitm.ac.in
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Table of contents
Sl. No.
Title
Page No.
1
Supporting methods
3
2
UV-Visible spectrum and TEM image of gold nanorods
5
3
Photograph of the set-up
6
4
Scattering spectra of GNR@GSH aggregates
7
5
Noise removal in the calculations of single particle PCDS
spectra.
8
6
Polar plots and dark field images of GNR aggregates
9
7
TEM images of gold nanorod aggregates
10
8
Description of supplementary video
10
2
Supporting methods
Chemicals. cetyltrimethylammonium bromide (99.9%) (CTAB), Tetrachloroauric acid
trihydrate
(HAuCl4·3H2O)
(99.9%),
sodium
borohydride
(99%,
Fluka),
(3-
mercaptopropyl)trimethoxysilane (95%) were purchased from Sigma Chemicals.
Sodium hydroxide (98%), L(+) ascorbic acid (99.7%), silver nitrate (99.9%), were
purchased from RANKEM, India. Chloroform (99.5%) was purchased from Thermo
Fisher Scientific India Pvt. Ltd.). Millipore deionized water (DI) (~18.2 MΩ) was used
throughout the experiments.
Synthesis of GNRs. GNRs were synthesized by seedless synthesis as previously
reported from our group [1]. Briefly, following chemicals were added sequentially, 50 mL
of 100 mM CTAB, 2.5 mL of 10 mM HAuCl4·3H2O, 325 µL of 10 mM AgNO3, and 350
µL of 100 mM ascorbic acid. Solutions were mixed by gentle shaking and no stirring
was done. Mixed solution was incubated for five minutes followed by the addition of 500
µL NaBH4 (1.67 mM dissolved in ice-cold DI water) was added. Then mixture was kept
undisturbed for overnight. After this, GNR solution was washed using DI water by
centrifugation at 7,500 rpm (20 min) once and twice at 12,000 rpm (20 min).
Concentration of GNR solution was calculated by previously reported formula [2] and
molarity of GNR solution was found to be ~0.2 µM.
Immobilization of GNR@GSH on glass slide. GNR@GSH were immobilized on a
glass slide by the pinpoint immobilization method previously reported from our group [2].
Briefly (3-mercaptopropyl)trimethoxysilane (0.2 µM in chloroform) was used for
immobilization of GNR@GSH on 1 mm thick ultrasonically cleaned glass slide
(SCHOTT). Immobilized GNR@GSH were covered with 0.145 mm thick Nexterion®
Clean room cleaned glass coverslip (SCHOTT) with DI water as surrounding medium.
Scattering micro-spectroscopy. Scattering microspectroscopic measurements were
performed using Cytoviva hyperspectral imaging system. Samples were illuminated
using high resolution, oil immersion dark field condenser (Cytoviva™). Scattered light
3
was collected using UPLFLN 100XOI2 oil immersion objective (Olympus, NA-1.3-0.6,
WD-0.2 mm). A L1090 - Halogen Lamp with Aluminum Reflector (International Light
Technologies Inc., USA) was used as source (400-1,000 nm). Optical images were
captured using Dagexcel-M Color Camera – Cooled (CCD, 2,048 × 2,048). Spectral
images were scanned using the Specim V10E line spectrometer (400-1,000 nm) at a
spectral resolution of ±1.3 nm. 3D glasses were purchased from http://www.ebay.in/. To
tackle with the decrease in intensity due to absorbance of polarizers, measurements
were performed at increasing shutter time as polarizers were added in the set-up. Since
RCP and LCP were having the same scattering spectrum, no specific correction was
done.
GSH induced aggregation of GNRs. It has been reported that assemblies of GNRs
formed upon interaction with glutathione (GSH) can exhibit chiral response [3]. To
construct chiral aggregates of GNR, as synthesized GNR30X10 were treated with GSH at
1:2 molar ratio of GNR:GSH. This mixture was vortexed vigorously for 5 min and
immobilized on 1 mm glass slides (SCHOTT) and covered with 0.145 mm coverslips
(SCHOTT) for imaging purposes. Minimal concentration of GSH was used to ensure the
availability of chiral and non-chiral GNRs to facilitate a comparison of both during the
experiment.
Image analysis. Image processing, particle tracking and RGB analysis was done using
NIH ImageJ [4]. MATLAB scripts were developed for polar mapping.
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Supporting information figures
Absorbance
725 nm
513 nm
400
600
800
1000
Wavelength (nm)
Figure S1: UV-Vis absorption spectrum of the GNRs used in this study. (a) Spectrum
has shown two absorption maximum at 513 nm and 725 nm corresponding to T-SPR
and L-SPR of GNR, respectively. Inset shows transmission electron microscopic (TEM)
image of these GNRs. Average dimensions of GNRs were found to be 30 (L) x 10 (D)
nm. Scale bar is 100 nm.
5
Olympus
BX41
Figure S2: Photograph of the set-up used for the illumination of samples with circularly
polarized light generated by 3D glasses. Olympus BX41 microscope was used for these
measurements. Magenta arrow shows the position of linear polarizer (D-POL) kept prior
to one of the 3D glasses. Green arrow shows a 3D glass kept on the top of linear
polarizer such that, quarter wave plate is on the top side and an axis of the additional
linear polarizer in 3D glass coincides with the axis of D-POL. Red arrow shows dark
field condenser and blue arrow shows the 100 X objective.
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Aggregate-2
Aggregate-1
LCP-RCP
Normalized scattering
1.0
1.0
RCP
RCP
0.5
0.5
0.0
400 600 800 1000
0.0
400 600 800 1000
1.0
LCP
1.0
LCP
0.5
0.5
0.0
400 600 800 1000
0.0
400 600 800 1000
0.6
0.6
0.3
0.3
0.0
0.0
-0.3
-0.3
-0.6
400 600 800 1000
-0.6
400 600 800 1000
Wavelength (nm)
Figure S3: Scattering spectra of aggregate ‘1’ and ‘2’ discussed in the Figure 4 of the
manuscript text. Top two rows show the spectra collected upon RCP and LCP
illumination, respectively. Bottom row shows corresponding PCD scattering spectrum
from Figure 4, just for comparison.
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(a)
(b)
CD
2.8
2.4
SD
Intensity (a.u.)
2.0
1.6
1.2
0.8
0.4
0.0
400
600
800
1000 400
Wavelength (nm)
600
800
1000
Wavelength (nm)
Figure S4: Scattering spectra collected with LCP and RCP dark field illumination for, (a)
GNR and (b) GNR aggregates. Standard deviations in their measurements (three
measurements) are shown in corresponding colors. PCDS spectra calculated as (LCPRCP) are also shown at the top. All the spectra are vertically translated without any
modification in the scale bar.
Noise removal in the calculations of single particle PCD spectra. Before getting into
this section, it is necessary to describe that CD/PCD implies, wavelength dependent
change in the absorbance of the entity under investigation. Hence, whenever one is
looking for difference in the signal from single particles upon LCP/RCP illumination, this
difference involves change in the curve shape instead of just change in the intensity of
the spectrum. Considering this, we have eliminated variations in the intensity by
normalization of spectra. These variations can be minor and can be due to changing
conditions such as replacing CPL filters or even minor disturbance that can be caused
by the scanner vibrations while collecting the spectra. To confirm that normalization
works, we have collected three spectra of the same sample (GNR aggregates) with LCP
and RCP illumination. Figure S4a and 4b shows spectra of two such particles. From the
spectra it can be seen clearly that, the spectra shown in Figure S4a are for single GNR
whereas spectra shown in Figure S4b are for an aggregate of GNRs. Each spectrum
was smoothed by weighed adjacent averaging. Then each spectrum was normalized
with minimum value as ‘0’ and maximum value as ‘1’. Normalization of scattering signal
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I(λ) was done using following formula. Here, I is the scattering intensity and λ is the
wavelength.
 
 
 
 


I   I 

min



norm 

 I  max  I 

min


I  
Where, I(λ)norm is normalized signal and I(λ)max and I(λ)min are maximum and minimum
scattering intensities, respectively. Then standard deviations (SD) for both the particle
spectra were calculated. These SD are plotted separately as shown in Figure S4. It can
be seen that, if there exist any variations in the intensity due to noise, those are almost
quenched by normalization. On the normalization scale (from 0-1), SD hardly goes
above 0.1. Next, subtraction of RCP signal from LCP signal gives the PCD spectrum.
Comparing the PCD spectrum of single GNR and GNR aggregate, chirality of GNR
aggregate can be noticed. This confirms that, just normalization of collected spectra
suffice the noise removal instead of collecting three spectra and averaging them.
1
0
315
45
270
90
225
2
135
180
0
315
45
270
90
225
135
180
Figure S5: Polar plots and dark field images of GNR aggregates ‘1’ and ‘2’ discussed in
the manuscript Figure 4.
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Figure S6: TEM images of gold nanorod aggregates.
Supporting notes
Description of multimedia view (Figure 2(g).mov):
A movie compiled from the images of GNR aggregates captured upon alternate RCP
and LCP illumination but linearly polarized collection. Some of the GNR aggregates
which exhibit plasmonic chirality are encircled by white circles and those which do not
exhibit plasmonic chirality are encircled by green circle. Blue square highlights the
CGNR and NCGNR for which data are shown in Fig. 3a of the main text.
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References
[1]
A. K. Samal, T. S. Sreeprasad, and T. Pradeep, J. Nanopart. Res. 12, 1777
(2010).
[2]
K. Chaudhari and T. Pradeep, Sci. Rep. 4, 5948 (2014).
[3]
Z. Zhu, J. Guo, W. Liu, Z. Li, B. Han, W. Zhang, and Z. Tang, Angew. Chem. Int.
Ed. 52, 13571 (2013).
[4]
W. S. Rasband, U. S. National Institutes of Health, Bethesda, Maryland, USA,
http://imagej.nih.gov/ij/ (1997-2012).
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