the effects of solvent purity on highly photoluminescent nanoparticles (cdse/zns quantum dots) used...

7/28/2019 The effects of solvent purity on highly photoluminescent nanoparticles (CdSe/ZnS quantum dots) used as biologic

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The Effects of Solvent Purity on Highly Photoluminescent Nanocrystals

(CdSe/ZnS Quantum Dots) Used as Biological/Biomedical Imaging Probes

Kevin Song

Carmel High School, Carmel, IN 46032

INTRODUCTION

Developments in the synthesis, characterization, and surface modification of nanoscale

structures have expedited their usage in biological and medical applications.1-4

Semiconductor

nanocrystals are widely used as probes for labeling cells and tissues owing to their intense,

tunable fluorescence.5-9

Quantum dots (QDs) are semiconductor nanocrystals whose

photophysical properties originate from the quantum confinement effect.10,11

QDs have been used

for many applications including light emitting diodes (LEDs), photovoltaic cells, fluorescent

dyes, antimicrobial agents, and cancer detection.12-16

The quantum confinement effect occurs

when the dimensions of a crystal semiconductor become smaller than the distance between a

valence band electron and its departed conduction band hole.10

A semiconductor nanocrystal

whose radius is smaller than the exciton Bohr radius forms a QD with discrete energy states.

Because band gap size is inversely related to nanocrystal size, QDs can be synthesized to emit

light at various wavelengths on the visible light spectrum.11

This quality, in addition to their

excellent luminescence and photostability, gives QDs a distinct advantage over traditional

organic dyes for biological imaging applications.

Figure 1. Band gap size is inversely related to nanocrystal size;QDs can be synthesized to emit at various wavelengths on the

visible light spectrum.2


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The small size and promising photophysical properties of QDs have made them attractive

optical labels in wide field single molecule fluorescence microscopy (SMFM) applications.18

However, due to the inconsistent quality of recently synthesized QD products, QDs that

fluoresced dimly and were unsuitable for SMFM were frequently encountered. The quality and

consistency of CdSe nanocrystals are normally greatly influenced by the presence of certain

impurities.19

The synthesis of semiconductor nanoparticles in reverse micelles was introduced in the

1980s.20-24

During that early synthetic era, the Steigerwald group discovered that poorly

crystalline CdSenanoparticles prepared in reverse micelles could be ripened and recrystallized in

n-Bu3P/n-Bu3PO mixtures.25

The phosphine-oxide component was found necessary to the

process. Shortly thereafter, Murray, Norris, and Bawendi introduced an organometallic synthesis

of CdE nanocrystals (E = S, Se, and Te) using n-octyl3P (TOP)/TOPO solvent mixtures, which

constituted a breakthrough in the preparation of semiconductor nanocrystals.26

Subsequently, tri-n-octylphosphine oxide (TOPO) and TOPO mixtures have been

employed as solvents in a wide range of nanocrystal syntheses.27-41 More recently, greener

syntheses of QDs employing non-coordinating solvents have emerged.42-48

However, TOPO

remains an important option for the synthesis of semiconductor nanocrystals.26-41,49-55 The likely

participation of adventitious TOPO impurities in nanocrystal growth has been recognized for

several years. TOPO is available from research-chemical suppliers in nominal 90%-purity

(technical) and 99%-purity grades (hereafter referred to as 90% TOPO and 99% TOPO,

respectively). Wang and co-workers identified such impurities in the main QD coordinating

solvent, TOPO, by31

P NMR.55

They incorporated these impurities into their thermal syntheses of

QDs, quantum wires (QWs), and quantum rods (QRs). These authors also concluded that one of

the impurities responsible for CdSe nanocrystal growth is di-n-octylphosphinic acid (DOPA).56


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Figure 2. CdSe/ZnS QDs fluorescing under a handheld UV lamp. As QD radiusincreases, so does the wavelength of light emitted by the QD, which results in a reddercolor of visible light.

11

Based on the studies of Wang and co-workers, it is hypothesized that increasing the purity

of TOPO and adding DOPA during sonochemical QD synthesis will cause more intense QD

fluorescence, more efficient photon emissions, and greater numbers of photons emitted per QD

molecule. The goal of this study is to test this hypothesis. QD products were sonochemically

synthesized from differing grades of solvent and were characterized by fluorescence

spectroscopy, UV-visible spectroscopy, and confocal fluorescence correlation spectroscopy

(FCS). This research could provide a good reference for the related synthetic reproducibility of

the growth of CdSe QDs.

MATERIALS AND METHODS

Materials

Tri-n-octylphosphine (TOP, 90%, Strem Chemicals), selenium powder (Se, 99.99%,

Aldrich), cadmium acetate (Cd(OAc)2, 99.9%, Aldrich), and hexadecylamine (HDA, 90%,

Aldrich) were used as received. Technical grade tri-n-octylphosphine oxide (TOPO, 90%) was

purchased from Strem Chemicals (batch number B4751105).

Purification of TOPO


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Distillation and recrystallization were performed to improve the purity of 90% technical

grade TOPO. Distillation had already been accomplished according to a literature procedure.57

For recrystallization, 30 g TOPO was transferred into a 500 ml Erlenmeyer flasks. Another flask

that contained acetonitrile (Aldrich) was kept warm in a water bath at 7075 C. Acetonitrile (7.5

ml) was added to the TOPO and the flask was gently hand-shaken. The supersaturated solution

was left to cool to room temperature. After the TOPO had precipitated, the flasks of solution and

acetonitrile were placed in an ice bath. To extract any remaining acetonitrile, the TOPO was

vacuum-filtered and stored in an unheated vacuum oven overnight. Two grades of purified

TOPO were assayed: (1) once-distilled and twice-recrystallized TOPO; and (2) undistilled, once-

recrystallized TOPO. TOPO straight from the manufacturer was also utilized for comparison.

Each grade of TOPO was used to synthesize QDs.

Sonochemical QD Core Synthesis

All syntheses of QDs were carried out with ultrasonic sonochemical methods as described

in literature.18

These methods have many advantages over conventional, thermally-intensive

techniques, including (1) lower temperature; (2) shorter time; and (3) greater control over

nanocrystal growth. Because sonication duration and power are directly controllable, it is

possible to stop the further formation and growth of nanocrystals once they have reached a

specific size that corresponds to a specific color of emitted visible light.

Figure 3. Schematic of sonochemical CdSe QD core synthesis and ZnS shelling.57

P

PO

NHNH2

PCH3

((CH2)7CH

3)3

((CH2)7CH

3)3

(CH2)15

CH3NHNH2

POCH3

PCH3Cd(OAc)

2+ Se

))) 18W, 15min

TOP, TOPO, HDACdSe

((CH2)7CH3)3

((CH2)7CH

3)3

(CH2)15

CH3

))) 8W, 10min

ZnC6H

10O

2S

4, TBP

CdSe

ZnS ((CH2)3CH3)3

I II


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For all QD CdSe core syntheses, 2.5 mL tri-n-octylphosphine (TOP) (Strem Chemicals)

was injected into a disposable borosilicate test tube containing 0.30 g selenium powder (Aldrich),

which was then sonicated at 10 watts (W) using a rod sonicator (Branson Ultrasonics) until all Se

powder was dissolved to form tri-n-octylphosphine selenide (TOPSe). 7.450 g TOPO (Strem

Chemicals) was combined with 2.35 g hexadecylamine (HDA) (Aldrich) and 0.240 g cadmium

acetate (Cd(OAc)2) (Aldrich) in a sidearm test tube. The mixture was melted using a heat gun on

low settings, and kept purged under argon gas while being sonicated at 14 W until 100 C was

reached. After the TOPO/HDA/Cd(OAc)2 mixture was cooled to 60 C, the TOPSe was added to

the mixture in the sidearm tube and sonicated at 18 W until the CdSe QDs turned dark red in

color (after about 15 minutes). Triplicates of 4 ml aliquots of these unshelled QDs were then

collected.

Figure 4. Tri-n-octylphosphine oxide (TOPO), themain coordinating solvent used in QD synthesis.

ZnS Shelling of QD Cores

To improve their fluorescence intensity, photon emission efficiency (quantum yield) and

shelf life (by protecting against photobleaching and oxidation), QD were shelled with ZnS, a

semiconductor material with a larger band gap than CdSe.58

Specifically, zinc ethylxanthate (0.1

g ZnC6H10O2S4) synthesized as described previously59

, and 2 ml tri-n-butylphosphine (TBP)

(Strem Chemicals) were vortexed in a disposable test tube, added to the test tube containing the


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QD cores, and sonicated for 10 min at 8 W. The temperature was maintained above 100 C, but

was kept lower than the maximum temperature reached (170200C) during QD core formation.

Incorporation of DOPA

QD products that incorporated DOPA were synthesized to compare the differences

between QDs made with and without DOPA. DOPA had already been synthesized following a

method described in the literature.57

To incorporate DOPA into QD synthesis, 0.04 mol% (0.021

g) DOPA was added to the TOPO/HDA/Cd(OAc)2 mixture in the side arm test tube before initial

melting and sonication, followed by CdSe core synthesis and ZnS shelling.

Figure 5. Di-n-octylphosphinic acid (DOPA), one of the TOPO impurities

believed to improve QD nanocrystal growth.

Hydrophilic Thin-Coating Method

Hydrophilic coatings were required for the QDs to be used in aqueous environments. A

method which had been previously described was used to functionalize the nonpolar QDs by

coating them in 3-mercaptopropionic acid (MPA).

62

Specifically, 0.15 g of shelled CdSe/ZnS

QDs was put into four disposable test tubes and dispersed in HPLC-grade chloroform (Fisher

Scientific). The QDs were then precipitated out of solution by adding a 50/50 acetone/methanol

solution, followed by centrifugation, disposal of supernatant, and drying under argon gas. After

the QDs were redispersed in chloroform, 0.15 g MPA (Aldrich) and a flake of 4-


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dimethylaminopyridine (DMAP) (Aldrich) were added to each test tube. The QDs were then

vortexed, centrifuged, removed of their supernatant, and dried under argon gas. 100 l of 5 M

NaOH (Aldrich) was added to one of the test tubes. The solutions in these test tubes were then

combined with each other while vortexing in between transfers. One ml deionized H2O was then

added to the QDs, followed by high speed centrifugation at 14,000 rpm for 30 minutes.

Biocompatible Lipopolymer Coating Method

QDs were functionalized with a lipopolymer coating as described previously.60

The QDs

were redispersed in HPLC-grade chloroform, precipitated, and dried in the same way as

described in the thin-coating method.61

DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine)

solution (96.8 L of 25 mg/ml stock, Avanti Polar Lipids) was combined with 600 l DPPE

PEG2000 (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[(polyethylene glycol)-2000])

solution. DSPEPEG2000 Maleimide (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-

[maleimide(polyethylene glycol)-2000]) solution (161 l of 1 mg/ml stock) was also added to the

lipid mixture, which was then poured into the test tube containing dry QDs and stirred to

redissolve the QDs in solution. The test tube was heated alongside another test tube that

contained 1 ml deionized H2O in an 85 C water bath for 15 minutes. The water in the heated test

tube was added to the QDs, which were then high-speed centrifuged at 14,000 rpm for 30

minutes.

Figure 6. DSPEPEG2000 Maleimide ((1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[maleimide(polyethylene glycol)-2000])) allows for QDsto bind onto lipids (picture courtesy of Avanti Polar Lipids).


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RESULTS

All QDs were synthesized using the same 90% technical grade TOPO (Strem batch

number. B4751105). The four compared QD products were as follows: (1) made from distilled,

twice-recrystallized TOPO with 0.04 mol% added DOPA (Product A); (2) undistilled, once-

recrystallized TOPO with 0.04 mol% added DOPA (Product B); (3) stock (undistilled,

unrecrystallized) TOPO with 0.04 mol% added DOPA (Product C); and (4) stock (undistilled,

unrecrystallized) TOPO without DOPA (Product D).

Fluorescence Spectra of Synthesized Samples

To obtain relative fluorescence intensities, the QD samples were each redispersed in

HPLC-grade chloroform at the same concentration and characterized using a Cary Eclipse

fluorescence spectrophotometer (Instrument Serial Number el02025045, Scan Software Version:

1.1(132)). The excitation wavelength was set to 400 nm.

Figure 7. Fluorescence spectra of synthesized QD products redispersed inchloroform. The Full Width at Half Maximum (FWHM) of the spectrum

for Product B was 33 nm and for Product A was 42 nm.


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Absorbance and Emission Spectra of Synthesized Samples

Absorbance data were obtained using an Evolution 600 UV-visible spectrophotometer

and emission spectra were obtained using the same Cary Eclipse fluorescence spectrophotometer

as described previously. To obtain quantum yield values, absorbance data and emission spectra

were obtained at three different concentrations (high concentration, medium concentration and

low concentration) of each QD Product (A through D) and a Rhodamine 6G standard in ethanol.

The absorbance values were taken at the excitation wavelengths for the CdSe/ZnS QDs (400 nm)

and Rhodamine (480 nm). The fluorescence spectra for all concentrations were integrated (using

Origin software) and plotted against their corresponding products absorbance values (Figure 8

to Figure 12).

Figure 8. The absorbance spectra of Product A at three concentrations

Figure 9. The absorbance spectra of Product B at three concentrations


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Figure 10. The absorbance spectra Product C at three concentrations

Figure 11. The absorbance spectra of Product D at three concentrations

Figure 12. The absorbance spectra of Rhodamine at three concentrations

FCS (Fluorescence Correlation Spectroscopy) of Hydrophilically Coated QD Samples

Fluorescence correlation spectroscopy was used to determine the hydrodynamic radii of

QDs with various hydrophilic surface coatings. Autocorrelation data were acquired using a Zeiss

ConfoCor2 fluorescence correlation spectrometer attached to a Zeiss Axiovert 200m Inverted

Microscope. The autocorrelation curve generated by the fluorescent nanoparticles diffusing in

bulk solution through a confocal excitation volume calculates the time it takes for a molecule to


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pass through the confocal volume. Through the use of this technique, the particle sizes of four

hydrophilic nanoparticles were measured. As seen in Figure 13-16, the autocorrelation curves

generated by various hydrophilic nanocrystal coatings can be linked to an overall diameter of the

quantum dot. As a comparision, the diffusion of Rhodamine dye was also measured.

Number 55, Carrier 5, Kinetics 1, REDQDS

10 100 1000 10000 100000 1e+006Time (s)

1.0

1.5

2.0

2.5

3.0

G ( t )

Figure 13. FCS of MPA-coated QDs made from undistilled, recrystallized TOPO and DOPA


10 100 1000 10000 100000 1e+006Time (s)

1.00

1.05

1.10

1.15

1.20G ( t )

Figure 14. FCS of Rhodamine 6G in water


10 100 1000 10000 100000 1e+006Time (s)

1.0

1.1

1.2

1.3

1.4

1.5

1.6

G ( t )

Figure 15. FCS of lipopolymer-coated QDs (Product A)


10 100 1000 10000 100000 1e+006Time (s)

1.0

1.1

1.2

1.3

1.4

G ( t )

Figure 16. FCS of MPA-coated QDs made from DOPA and purified TOPO


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ILLUSTRATIONS

Figure 17. Relative fluorescence quantum yields of synthesizedQD products (redispersed in chloroform).

Figure 18. Mean (n = 6 FCS runs of 10 scans per sample) photon counts permolecule for synthesized QD Products A and B (coated in either MPA or lipopolymer)and Rhodamine 6G. The total photon acquisition time was 600 s.


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QD-linked Lipid Probe Tracking

Figure 19. Snapshots of a fluorescent QD-linked lipid probe were taken by

another researcher (snapshot time interval set to 41 msec) and the probescoordinates were plotted. Brownian motion was detected.

DISCUSSION

Solvent Impurities Result in More Intense Fluorescence

QDs made from undistilled, once-recrystallized TOPO and DOPA (Product B) had the

brightest fluorescence (Fig. 7). The second brightest product was synthesized from distilled,

twice-recrystallized TOPO and DOPA. The QDs synthesized from commercial TOPO (Strem

batch number B4751105) and DOPA had the broadest emission peaks, and even fluoresced from

420 to 550 nm, whereas the other dots did not fluoresce in this range. All four QD products had

their emission peaks centered on 600-620 nm. The FWHM of the QDs made from undistilled,

once-recrystallized TOPO and DOPA (Product B) was 33 nm. This FWHM was less than the

FWHM of 42 nm for the QDs (Product A) made from distilled, twice-recrystallized TOPO and

DOPA. This data indicated that solvent impurities resulted in more fluorescent QDs. The fact

DISTANCE (m)

DISTANCE

(m)


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that more purified (distilled, twice-recrystallized) TOPO and DOPA resulted in less luminescent

QDs than less purified (undistilled, once-recrystallized) TOPO and DOPA, suggested that the

presence of other unidentified impurities was beneficial.

Solvent Impurities Result in Better Quantum Yields

Quantum yield is defined as a function of photon emission divided by photon absorbance.

After the relative quantum yields of the four QD products were obtained, the efficiencies of the

QD products proton emissions were compared. The method used to obtain quantum yields

required taking both absorbance and emission spectra for the sample solutions16

, but was adapted

so that multiple concentrations of each sample (QDs redispersed in chloroform and Rhodamine

6G in ethanol) were analyzed (Figure 8-12). The following formula, adapted from a previously

described method, was used:62

In this equation, is the quantum yield, the gradient represents the slope of the least

squares regression line of the plot of integrated fluorescence against absorbance, Trepresents the

test sample, Srepresents the standard sample, and is the refractive index of the solvent. A least

squares regression line was calculated (using Microsoft Excel software) for each QD product and

Rhodamine. The slopes of these lines, the quantum yield of the Rhodamine standard (0.95), and

the refractive indices of chloroform (1.446) and ethanol (1.361) were used to calculate the

products relative quantum yields.


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Table 1. Quantum Yield of QD Products (QY of Rhodamine 6G Standard = .95)

Sample Gradient Quantum Yield (%)

A 40905 4.39

B 207380 22.24

C 17590 1.89

D 27938 3.00

Rhodamine 1000000 95.00

The quantum yield of QDs synthesized from undistilled, once-recrystallized TOPO and

DOPA (Product B) was about five times as efficient as that of the QDs made from distilled,

twice-recrystallized TOPO and DOPA (Product A) (Fig. 17). The quantum yield of the QDs

made from only stock TOPO (no DOPA) was higher than the quantum yield of the QDs made

from stock TOPO and DOPA. The results showed that the recrystallization of TOPO and the

addition of DOPA made the greatest difference in quantum yield.

TOPO Impurities Result in More Photons Emitted per QD

The QD samples were characterized by a Zeiss ConfoCor2 microscope using

fluorescence correlation spectroscopy (FCS). Since FCS cannot be used to characterize QDs that

are redispersed in chloroform, it was necessary to hydrophilically coat them using either one of

the MPA surface-exchange or lipopolymer encapsulation methods described previously.14,15

Due

to their greater brightness and quantum yield, only Products A (distilled, twice-recrystallized

TOPO and DOPA) and B (undistilled, once-recrystallized TOPO and DOPA) were

functionalized with MPA and lipopolymer hydrophilic coatings. The functionalized QD products

were analyzed by FCS six times (n = 6). A sample containing Rhodamine 6G in water was also


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characterized in order to compare QD photon counts with that of a conventional fluorescent

organic dye.

The MPA-coated QDs synthesized from undistilled, once-recrystallized TOPO and

DOPA had a photon count per molecule of about 650, which was 13 times higher than that of

Rhodamine 6G (Fig. 18). All QD samples, except the lipopolymer-coated ones synthesized from

distilled, twice-recrystallized TOPO and DOPA, emitted greater numbers of photons than

Rhodamine 6G. The MPA-coated dots were brighter than the lipopolymer-coated QDs. It is

intriguing, perhaps, that although Rhodamine 6G has a much higher quantum yield than the QDs,

the QDs emit more photons per molecule. This can be explained by the QDs higher extinction

coefficient that enables them to recover much faster than Rhodamine 6G does.

Biofunctionalized QDs Synthesized from DOPA Track Single Lipid Movement

Lipopolymer-coated QDs from product B were used to track lipid movement on a model

bilayer with single particle tracking (SMFM) as described previously.60

By incorporating DSPE

PEG2000 Maleimide into the lipid-lipopolymer coating mixture, lipopolymer-coated QDs could

be linked onto lipids with thiol headgroups (SH-lipids), which were incorporated into a

phospholipid lipid bilayer.12

Figure 20. Lipopolymer-coated QD-linked lipid probe.63


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A series of still snapshots of a moving QD-linked lipid were taken by another researcher

and the QDs position coordinates were tracked over 41 millisecond intervals when its

movement was plotted on a coordinate grid (Fig. 19). QDs synthesized with DOPA and

undistilled, once-recrystallized TOPO were more visible than QDs made from unadulterated

(undistilled, unrecrystallized) stock TOPO when viewed with SMFM microscopy, in excellent

agreement with the spectroscopy results.

CONCLUSIONS AND FUTURE WORK

In this study, the effects of TOPO purity and DOPA addition on sonochemically

synthesized CdSe/ZnS QDs were observed by comprehensive measurements including

fluorescence spectroscopy, UV-visible spectroscopy, and confocal fluorescence correlation

spectroscopy characterization. Up until now, traditional TOPO (unpurified and with no solvent

additives) had been used to produce mediocre products of QDs with their quality varying from

solvent batch to solvent batch. The limitations of traditional QDs synthesized from stock TOPO

of an average batch were (1) extremely large FWHMs; (2) the inability to fluoresce under a

conventional handheld UV lamp; and (3) very low photon counts per molecule. This study has

provided strong evidence that impurities such as DOPA resulted in brighter, more efficient QDs.

This work has also indicated an ability to produce higher-quality QDs for applications in industry,

biological imaging, and medical imaging and diagnosis.12-16

Future Directions

The effects of the presence of DOPA on QDs could be expanded in future studies

involving different concentrations and degrees of additive purification by distillation or

recrystallization. Characterization of TOPO and its impurities could be accurately achieved by

utilizing gas chromatography-mass spectrometry (GC-MS) or31

P NMR.13

Also, atomic force

microscopy18

(AFM) could be used to compare the isotropies of QDs synthesized from stock


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TOPO and QDs synthesized from TOPO with various incorporated additives. It was reported on

September 28, 2010, that scientists at the University of Illinois have developed a nanoneedle that

releases quantum dots directly into the nucleus of a living cell when a small electrical charge is

applied. Their study generated extremely small-size and high quality QDs that have the potential

to be utilized as in vivo optical imaging probes and in a variety of research applications.


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