1. BACKGROUND
In 1996, Garetz and coworkers discovered that supersaturated
solutions can be induced to crystallize when illuminated with a high-
intensity laser beam and named this process non-photochemical laser
induced nucleation (NPLIN). Although many other ways of causing
supersaturated solutions to crystallize were already known, NPLIN is
especially significant because it represents a non-chemical way of
forcing different polymorphs of the solute to emerge for some
substances, such as glycine. Since different polymorphs of a substance
exhibit differences in properties such as solubility, density and
bioavailability, the ability to select a particular polymorph of a
substance bears great importance in the pharmaceutical industry.
What determines which polymorph is formed in NPLIN is the
polarization state of the laser beam, a process called polarization
switching. To explain this phenomenon, Garetz et al. hypothesized that
NPLIN was based on an optical Kerr alignment: the electric field of the
electromagnetic wave (light) induces a certain kind of alignment in the
molecules, causing them to organize and eventually form the crystal.
MARIA ANDREEA DIMITRIU
BS Biomolecular Science 2018
Faculty: Bruce Garetz, Janice Aber
LASER-INDUCED CRYSTALLIZATION IN THE
PRESENCE OF GOLD NANOPARTICLES
Figure 2: Pictures of 4 different crystals after separation and drying
ABSTRACT
The hypothesis Garetz and coworkers reached, that NPLIN is based on
an electric-field-induced alignment, suggests that if one can enhance
the strength of the electric field of the laser beam, one could observe
NPLIN at lower light intensities. This idea constitutes the basis of our
current experiment, which studies the effect that gold nanorods have
on solutions of 1.5 supersaturation of glycine dissolved in aqueous
sodium citrate when they are shot with linearly-polarized light of
wavelength of 1064 nm in the near-infrared spectral region. We have
obtained gold nanorods that exhibit plasmon resonances near this
wavelength (resonance wavelengths ranging from 660 to 980
nanometers) that magnify the strength of the local electric field. This
local electric field amplification makes us expect that samples to which
gold nanorods were added will exhibit a higher laser-induced
nucleation rate than the ones not containing gold nanorods, and that
the intensity threshold for nucleation will be lower in the presence of
gold nanorods.
METHODS AND TECHNIQUE
This experiment’s goal is to draw conclusions regarding differences between
spontaneous and laser-induced nucleations. Nucleation is a stochastic process,
so large populations must be analyzed in order to obtain significant statistics.
Thus, for each kind of solution analyzed several 30-sample batches are
prepared. All samples prepared for this experiment are solutions of
supersaturation between 1.49 and 1.51 made with 0.675 grams of glycine and
2.000 grams of solvent. The three kinds of solutions being analyzed are:
aqueous solutions (where the solvent is pure water), solutions whose solvent is
a 2mM aqueous solution of sodium citrate, and solutions made with sodium
citrate to which 100 µL of gold nanorods solution are also added.
To prepare solutions, we use test tubes of 1.3 centimeter diameter with caps.
First, the tubes and caps are cleaned: they are filled with a 1% solution of
Micro90 detergent, placed in a Crest Tru-Sweep Ultrasonic Cleaner and
sonicated for 60 minutes at 40°C and power 9. Then, the tubes and caps are
rinsed 10 times with UV-filtered water and placed in a Fischer Isotemp Oven to
dry overnight. The clean tubes are used to prepare the supersaturated
solutions: the necessary quantities of each substance are added in the tube, the
tube is capped and numbered. Next the samples are sonicated at 60°C and
power 5 for around 24 hours, until the glycine dissolves. Then, the sonicator is
turned off and all tubes are left inside to slowly cool to room temperature
overnight, forming the supersaturated metastable solutions. Solutions are aged
for a week during which they are checked for spontaneous nucleation. After a
week, the solutions that have not spontaneously nucleated are illuminated with
the laser.
The optical setup that was used to shoot the samples is shown in Figure 1. The
laser used is a Quanta-Ray® Nd:YAG laser, the wavelength is 1064 nanometers,
the frequency is 10 Hz; each pulse’s length is 9 nanoseconds. The apertures in
the setup shrink the beam to protect the polarizer, which allows only linearly-
polarized light to pass. The lens was added to increase the intensity of the
beam. Samples are placed at 32 centimeters from the lens, where the laser
intensity is around 350 mW/cm2, and each sample is shot for 1 minute at
maximum energy, 70 Joules per pulse. The mean power obtained in these
conditions is around 2.5 Watts.
Solutions shot are being checked for nucleation for 24 hours after the shooting,
which is the time frame during which it can be assumed that the nucleation is
laser-induced. All crystals that are formed are separated from the remaining
solution, dried using a vacuum pump, and stored. Finally, the crystals are
analyzed using single-crystal or powder X-ray diffraction to determine details
regarding the polymorph of each crystal.
L1 = Laser; A1 = Aperture 1; A2 = Aperture 2; P = Polarizer; L2 = Lens; T = Test Tube
Figure 1: Optical Setup
Refrences:
[1] Sun X. 2008. Polymorph Control in Non-Photochemical Light-Induced Nucleation and
Optical Kerr Study [dissertation]. Polytechnic University.
[2] Garetz, B.A., & Aber, J. E., & Goddard, N.L., & Young, R.G., & Meyerson, A.S. (1996).
Nonphotochemical, Polarization-Dependent, Laser-Induced Nucleation in Supersaturated
Aqueous Urea Solutions. Physical Review Letters, 77, 3475-3476.
[3] Matic, J., & Sun, X., & Garetz, B.G. (2005). Intensity, Wavelength, and Polarization
Dependence of Nonphotochemical Laser-Induced Nucleation in Supersaturated Aqueous
Urea Solutions. Crystal Growth & Design, 5, 1565-1567.
RESULTS AND FUTURE WORK
Batch # Solvent SN # Shot NPLIN % NPLIN
1 Water 2 13 6 46%
6 Citrate 2 25 12 48%
7 Citrate 3 27 9 33%
8 Citrate 9 20 7 35%
9 Citrate 6 24 17 71%
10 Citrate 4 23 12 52%
11 Citrate 12 15 11 73%
12 Citrate 4 25 13 52%
13 Citrate 3 25 15 60%
14 Citrate 10 15 13 86%
15 Citrate 13 12 7 75%
Conclusions regarding the nucleation rates of the solutions can be drawn from
observation of a large population. The table above shows details regarding the
number and type of nucleations for a total of 330 samples. First, it is important
to understand that metastable solutions are extremely sensitive to
environmental factors such as temperature or movement and conclusions
should not be drawn without taking this into account. While most batches
showed a relatively low number of spontaneous nucleasions (SN): 2 to 6,
batches 11, 14 and 15 exhibited a much higher number. We concluded that
those batches were disturbed by constructions taking place in the building, as
the batches had just been taken out of the bath at the time the lab was affected
by construction. The same batches also exhibit a higher percentage of laser-
induced nucleation (NPLIN), which would confirm our suspicions. Excluding
those batches, we can conclude that at the intensity of 350 MW/cm2 (measured
at the front of the tube), between 50 and 70% of the samples made with
aqueous sodium citrate will nucleate when illuminated with the laser. Also
taking into consideration previously reported results, it is apparent that samples
made with pure water exhibit a slightly lower laser-induced nucleation rate
(below 50%) at the same intensity. From the same data it is observable that as
samples age, they become less probable to nucleate when illuminated by the
laser (batches 7 and 8 were the oldest and had the lowest nucleation rates).
Further work would involve performing X-ray diffraction on all crystals obtained
for each batch, in order to be able to establish polymorph formation trends
with respect to the nucleation type, as well as any possible effect that the gold
nanorods would have on the polymorph formation, which would be noticeable
when comparing results for samples containing gold nanorods to those of
regular samples.