Calorimetric and spectrophotometric investigation of PLGA nanoparticles and their complex with DNA

Abstract

The calorimetric investigation of non-coated and chitosan-coated PLGA nanoparticles (NP) shows that at initial temperatures
of heating particle swelling takes place what results in an internal architectural change at lower than physiological temperature.
It has shown that the temperature of NP tightness perturbing depends on solvent polarity: as more polar is the solvent more
stable are particles. The break of existing bonds in NP shell is accompanied with heat absorption peak which undergoes significant
changes depending on heating rate. In the wide pH 2–8 interval in transition temperature no changes occurred. The obtained
results show that such NP could be used in acidic area for drug transfer, which gives possibility to take medicine orally.
It was shown that DNA attaches only to chitosan-coated NP. The optimal ratio for DNA loading onto the NP was found to be 7:1
(WNP/WDNA).

Full text

Calorimetric and spectrophotometric investigation of PLGA
nanoparticles and their complex with DNA
Mariam Khvedelidze ÆTamaz Mdzinarashvili Æ
Tamar Partskhaladze ÆNoha Nafee ÆUlrich F. Schaefer Æ
Claus-Michael Lehr ÆMarc Schneider
Received: 28 October 2008 / Accepted: 26 May 2009 / Published online: 13 August 2009
ÓAkade
´miai Kiado
´, Budapest, Hungary 2009
Abstract The calorimetric investigation of non-coated
and chitosan-coated PLGA nanoparticles (NP) shows that
at initial temperatures of heating particle swelling takes
place what results in an internal architectural change at
lower than physiological temperature. It has shown that the
temperature of NP tightness perturbing depends on solvent
polarity: as more polar is the solvent more stable are par-
ticles. The break of existing bonds in NP shell is accompa-
nied with heat absorption peak which undergoes significant
changes depending on heating rate. In the wide pH 2–8
interval in transition temperature no changes occurred. The
obtained results show that such NP could be used in acidic
area for drug transfer, which gives possibility to take medi-
cine orally. It was shown that DNA attaches only to chitosan-
coated NP. The optimal ratio for DNA loading onto the NP
was found to be 7:1 (W
NP
/W
DNA
).
Keywords Calorimetry (DSC) Nanoparticles 
Spectrophotometry Drug delivery systems PLGA 
Nanoparticles DNA complexes
Introduction
Modern pharmaceutical approaches have allowed to iden-
tify many potent drugs. Unfortunately, many of those show
a small bioavailability because of their limited aqueous
solubility. Hence, NP offer the potential to overcome not
only this obstacle but also promise smaller amounts of
drugs delivered to specific cells and tissues [1–3]. Also the
perspective of using NP is very interesting when the
problem is to deliver the therapeutic agents to the damaged
organ or tissue unaltered, crossing natural barriers which
are the first obstacle for drug delivery. For a broad appli-
cability the consideration of diverse environmental condi-
tions such as high acidic pH, unfavorable metal ions, active
enzymes, etc. are essential. Nanotechnology is considered
to inherit a huge potential for drug delivery, since it offers a
suitable transportation of anticancer agents, antihyperten-
sive agents, immunomodulators, hormones, and macro-
molecules such as proteins, peptides, antibodies or genes
[1–12].
Recently, NP with a therapeutic agent of interest
encapsulated in their polymeric matrix or adsorbed or
conjugated onto the surface [1,13–28] can be administered
orally or injected locally [23,25,29]. One of the most
common NP is made of poly (lactic acid) (PLA) [30], poly
(glycolic acid) (PGA), and their copolymers poly (lactic-
co-glycolic acid) (PLGA) [25,31–39]. Such polymers have
controllable biodegradability, excellent biocompatibility,
high safety, they are non-toxic, and they are restorable
through natural pathways [34,40–46]. One disadvantage of
M. Khvedelidze T. Mdzinarashvili (&)T. Partskhaladze
Department of Physics, Faculty of Exact and Natural Sciences,
Ivane Javakhishvili Tbilisi State University,
3, Chavchavadze Ave., 0128 Tbilisi, Georgia
e-mail: mdz@tsu.ge
M. Khvedelidze T. Mdzinarashvili
Institute of Molecular Biology and Biophysics,
12, Gotua Str., 0160 Tbilisi, Georgia
N. Nafee U. F. Schaefer C.-M. Lehr
Biopharmaceutics and Pharmaceutical Technology, Saarland
University, Postfach 151150, 66041 Saarbru
¨cken, Germany
M. Schneider
Pharmaceutical Nanotechnology, Saarland University,
Postfach 151150, 66041 Saarbru
¨cken, Germany
123
J Therm Anal Calorim (2010) 99:337–348
DOI 10.1007/s10973-009-0137-x

existing delivery systems is the limitation by using organic
solvents and relatively harsh formulation conditions.
PLGA NP are generally formulated using emulsion solvent
evaporation or by solvent displacement techniques [33],
which induce some problems with limited core loading, i.e.,
\15%, the variable burst release of entrapped drug and
organic solvent residues [47]. One of the main drawbacks
of colloidal carriers is that they tend to agglomerate during
storage. Therefore it is important to determine the struc-
tural changes which take place in NP shell. There are lit-
erary data indicating that the drug entrapped in PLGA
matrix is released at a sustained rate through diffusion of
the drug in the polymer matrix and by swelling and deg-
radation of the polymer matrix [32,47,48], though there
has not been studied completely the mechanism by which
the drug entrapped in such NP releases in time.
Hedley et al. have demonstrated protection of DNA from
nucleases when encapsulated into PLGA microspheres [49].
By changing the copolymer composition and molecular
weight it is possible to vary the release of encapsulated drugs
from PLGA NP from days to months [1,33,35,50–55].
There are literary data confirming that DNA can be con-
densed on dendrimers [56,57], cationic peptides [58,59],
cationic polymers [60–67], cationic lipids [68–72], as well as
on liposomes [47,73–80]. DNA could be transferred to the
cells encapsulating in or by adsorption onto the particles
surface [54,55,69,81–84]. These properties are the reason
for PLGA being approved by the FDA [33].
Chitosan is a promising and often used candidate for
surface modification due to its biocompatibility and its
positive charge [85]. Chitosan-modified PLGA should
adsorb better than other lipophilic polymer derivatives,
because of its hydrophilicity, and improve transfection rate
of the particle–DNA complexes in vitro and also in vivo
experiments [86].
For successful application of NP as gene delivery sys-
tems their interaction with the payload is essential.
Therefore a fundamental study was performed regarding
NP and their possible biological active complexes. From
this point of view it is important to investigate the influence
of each possible environmental parameter on NP–DNA
complexes. The goal of our study was the biophysical
investigation of the physico-chemical properties of PLGA
NP. Especially, the examination of the influence of envi-
ronmental conditions (pH, temperature, polarity of the
solvent) on the NP behavior and their interaction with
DNA was investigated. For this reason two types of NP
have been chosen to study: non-coated and chitosan-coated
NP with the main size 148.2 nm and 146.8 nm, respec-
tively. These two almost the same sizes nanoparticle (but
with different charges) have been chosen for study the
influence of potential on stability and interaction possibility
with DNA at different environment conditions.
Materials and methods
Non-coated and chitosan-coated PLGA NP were made
using solvent evaporation methods [83,87–89]. In brief,
PLGA 70:30 was dissolved in 5 mL ethyl acetate at room
temperature. The organic phase was added dropwise to an
equal volume of the aqueous phase, containing the stabi-
lizer PVA (2.5% w/v) for PLGA particles and PVA and
chitosan for chitosan-coated particles (cNp), under stirring
using a magnetic stirrer, at 1000 rpm, for 1 h, at room
temperature. The emulsion was then homogenized (Ultra-
Turrax T25, Janke & Kunkel GmbH & Co-KG, Germany)
at 13,500 rpm for 10 min. Nanoprecipitation was per-
formed by adding MilliQ water dropwise under gentle
stirring to obtain a final volume of 50 mL. Stirring is
continued overnight at room temperature to get rid of the
organic solvent. The size distribution of the particles was
determined using photon correlation spectroscopy. The
non-coated PLGA nanoparticle (nNP) size was found to be
d=148.2 nm (PI =0.03, charge -8.6 mV). In the case of
chitosan-coated PLGA NP (cNP) the size was 146.8 nm
(PI =0.12, charge ?39.98 mV).
Calf thymus DNA (Sigma) has been chosen to investi-
gate the particles interaction with nucleotides. All
degrading influences such as contact of the sensitive DNA
with extreme pH values or organic solvents during the
preparation process were avoided because DNA was added
after all other preparation steps were finished.
The study of the thermodynamic features of PLGA NP
and the interaction between DNA and the NP were carried
out using a precise DASM-4A microcalorimeter (Russian
Academy of Science, Pushchino, Russia), which belongs to
high sensitivity type heat flow calorimeters [90]. In addi-
tion, this device allows to carry out experiments with low
rate of temperature scanning and therefore to perform quasi
equilibrium measurements. The spectrophotometric inves-
tigation was done using the spectrophotometer HEkIOS b
(Thermospectronic, Thermo Fisher Scientific, USA). The
centrifugation of NP was accomplished using the Beckman
Coulter
TM
, Allegra
TM
64R Centrifuge and particle sedi-
mentation was accomplished at 18,000 rpm (23183 g). The
device of turbidity was constructed in Tbilisi State Uni-
versity by us, where as a source of light it is used blue
light-emitting diode (with wavelength k=480 nm) and
the detector of light is photomultiplier.
Results
For nanoparticle application in biological systems it is
important to investigate their stability under different
environmental conditions such as temperature, pH, various
salts, etc. The thermodynamical properties of NP have been
338 M. Khvedelidze et al.
123

studied to gain some insights about nanoparticles’ proper-
ties using supersensitive differential microcalorimetric
method.
To calculate nanoparticle specific heat capacity it is
necessary to know nanoparticle partial volume. According
to nanoparticle weight, diameter and its shape, we calcu-
lated the partial volume. NP in water were situated in
suspended condition and had a spherical shape with
approximately 147 nm diameter [89]. This allows to cal-
culate the nanoparticle volume which turned out to be on
average 1.7 910
-15
cm
3
. Based on nanoparticle mean
weight which is 2.88 910
-15
g, finally, it is possible to
calculate the nanoparticle partial volume which turned out
to be m=0.59 cm
3
g
-1
(or density q=1.695 g cm
-3
).
In Fig. 1the chitosan-coated PLGA nanoparticle (a) and
non-coated nanoparticle (b) suspension microcalorimetric
fits are shown. The heating rate was 2 °C min
-1
.Asitis
seen from calorimetric recording the curves have difficult
form, but for both particles have the same appearance.
Therefore hereinafter we give only the results for the
thermodynamic parameter investigation for one of the NP,
as the results were basically identical for equal conditions.
From the microcalorimetric curve four temperature inter-
vals can be concluded: (a) 10–23 °C; (b) 23–35 °C;
(c) 35–130 °C; (d) 130–140 °C. The heat capacity increase
in 10–23 °C temperature interval during temperature rise is
due to the fact that the volume of nanoparticles’ increases
(Fig. 1). As the DASM microcalorimeters measure the heat
capacity of investigated solution with constant volume, so
even the negligible rising of nanoparticles’ volume causes
the changes in heat capacity value of nanoparticles’ solu-
tion which we have observed in our experiments. As it can
be seen the subsequent increasing of temperature leads
to swelling and the stretching of particles’ surface and
in 23–35 °C temperature interval the particles’ ordered
structure change occurs and its core becomes reachable for
the solvent. The phase transition temperature T
m
for the
chitosan-coated NP is 30 °C and in the case of non-coated
NP is 28 °C. The enthalpy of this peak of heat absorp-
tion in both cases is in order of 2 J g
-1
. Furthermore, at
elevated temperatures (T
m
[50 °C) the monotonously
decreasing curves indicate that the particles start to
aggregate (Fig. 1). Further increasing of the temperature
leads to particles’ aggregate destruction (above 120 °C)
which appears in sharp increase of particles’ heat capacity.
It was carried out turbidity measurements of NP with
temperature (non-coated; PLA/PLG 70:30). The suspen-
sion of NP was placed in the glass tube with length 10 cm,
which was heated by heater which was surrounded around
tube. The measurement of temperature was carried out by
mercury thermometer, which with good thermal contact
was fixed on the glass tube. On the Fig. 2it is given
dependence of suspension transparency changes on tem-
perature during scanning the temperature, from which it is
obvious that while increasing the temperature of NP in
15–35 °C temperature interval the intensive suspension
transparency increase takes place (heating of suspension up
Fig. 1 Dependence of the specific heat capacity on temperature: ain
the case of chitosan-coated PLGA nanoparticles; bin the case of non-
coated PLGA nanoparticles. The heating rate was 2°min
-1
. Solvent
was bidistilled water, pH 5.0
Fig. 2 Dependence of noncoated nanoparticles’ suspension transpar-
ency on heating temperature
Calorimetric and spectrophotometric investigation of PLGA nanoparticles and their complex with DNA 339
123

to higher temperatures ([50 °C) was not achieved for this
time due to construction of device).
Spectrophotometric data revealed that the so-called
Rayleigh scattering (light intensity is proportional to k
-4
)
is the same for both, ‘‘native’’ particles and particles after
heating them till 150 °C and cooled then down (Fig. 3). In
other words, the absorption spectrum of the destroyed
chitosan-coated NP, obtained by heating up to 150 °C, did
not differ from the spectrum of the ‘‘native’’ ones. The
same results were obtained for non-coated NP.
To determine whether the NP melting is a kinetic pro-
cess or not, the calorimetric experiments with NP suspen-
sions at different heating speeds (0.5, 1, 2, 4 °C min
-1
)
have been carried out (Fig. 4). These experiments show
that the thermodynamic parameters such as T
m
and the
particle break-up enthalpy (the area of the peak in
23–35 °C temperature interval) depends on particle heating
rate. In particular, the higher the speed of heating, the
higher is the particle transition temperature T
m
and their
melting enthalpy (Table 1).
In addition, the experiments have been carried out with
the so-called nanoparticle, annealing by temperature, where
NP were heated only till 23 °C—the temperature where the
NP internal architecture is starting to modify (Fig. 5), and
then the calorimeter was turned to cooling regime. The peak
is restored at lower temperature (T
m
=25 °C) after particle
reheating and enthalpy of this peak is also decreased
(approximately three times). According to Fig. 5the parti-
cles change occurs at lower temperature (the particles,
heated until 23 °C, were already modified) than in this
case of heating ‘‘native’’ once, where the T
m
=30 °C (see
Fig. 1a).
For practical application it is important to determine
the storage stability of the NP. Especially, the storage in
suspension for these biodegradable samples is of high
interest. For this reason the calorimetric experiments with
nanoparticle prepared 3 months before and stored at 4 °C
in refrigerator were carried out. The result was compared
with the same experiment performed with freshly pre-
pared particles. Figure 6shows calorimetric curves for
freshly prepared non-coated NP (line 1) and the same
Fig. 3 The light absorption spectra of chitosan-coated PLGA nano-
particles (the solid line) with 1.238 mg mL
-1
concentration and for
denatured particles with the same concentration (the dash line).
Solvent was bidistilled water, pH 5.0
Fig. 4 Dependence of the specific heat capacity of non-coated PLGA
nanoparticles on temperature at different heating rates: 1—0.5, 2—1,
3—2, 4—4 K min
-1
Table 1 Dependence of transition temperature (T
m
) and melting
enthalpy (DH) of non-coated PLGA nanoparticles on the different
heating rates
Heating rate
V/°C min
-1
Transition temperature
T
m
/°C
Melting enthalpy
DH/J g
-1
0.5 26.5 1.87
1 27.9 1.92
2 29.5 1.98
4 31.7 2.25
Fig. 5 Dependence of the specific heat capacity of non-coated PLGA
nanoparticles on temperature: first heating till 23 °C; second is the
totally heating up to 150 °C
340 M. Khvedelidze et al.
123

particles after 3 months (line 2). As it can be seen from
this figure storage time influences only particle aggrega-
tion process, in other words, the influence of time on
NP becomes apparent in an attempt to increase particle
aggregation (they aggregate at lower temperature: in
40–110 °C range).
It is known that in creation of nanoparticle existence of
hydrophobic forces play an important role. Therefore, we
changed the solvent’s polarity to investigate the solvent–
particle interaction. The calorimetric experiments in 10%
ethanol solution lead to a decreased the transition tem-
perature to T
m
=24 °C (Fig. 7). In contrast to water, in
ethanol no aggregation was observed. Moreover, because
of the reduced solvent hydrophobicity no particle aggre-
gation process was observed.
As it was mentioned above, under the influence of
temperature, at first NP widening (10–25 °C) takes place,
which is followed by the particle cracking process at
T
m
=30 °C. Hence the particle transition temperature has
to be depended on the environmental pressure. In all our
calorimetric experiments the samples were situated under
6–7 atmosphere pressure to avoid the solution boiling
process during the heating. To find out if the pressure
influences on widening process, the experiments have been
performed at 1.5 and without excess pressure. No differ-
ences were obtained.
We consider that the knowledge of polymer solution
behavior is necessary to improve understanding of micro
sphere formation and drug micro encapsulation. Obviously,
the purpose of these experiments was to determine the role
of deionized water in the stability of NP. Our results
emphasize that using deionized water is not so necessary
for stability of nanoparticle. For this reason other param-
eters such as the solvent’s pH were investigated. Figure 8
shows microcalorimetric study of NP in buffers of different
acidity ranging from pH 2 to 8.2. The buffer molarity was
chosen not too low (0.02 M Na
2
HPO
4
and 0.01 M citric
acid) to obtain a sufficient buffer capacity for maintaining
the solution’s pH during the heating process in the wide
temperature interval (10–150 °C). These data are compared
to the calorimetric data of NP suspension in pure bidistilled
water.
To determine the interaction of PLGA non-coated NP
and chitosan-coated NP with DNA the calorimetric, ultra-
centrifuge and spectroscopic methods were applied. The
calorimetric experiment was carried out using mixture of
DNA and chitosan-coated NP solution, which shows that
the heat absorption peak is constricted in the case of DNA
presence in particles’ solution, what biophysically means
that interaction between them takes place (Fig. 9). In this
experiment the nanoparticle/DNA ratio was 1200:1 (w/w).
At higher DNA concentrations aggregates appear in solu-
tion which made the calorimetric investigation impossible.
Fig. 6 Dependence of the specific heat capacity of non-coated PLGA
nanoparticles on temperature: 1—fresh prepared nanoparticles; 2—
the nanoparticles after 3 months
Fig. 7 Dependence of the specific heat capacity of chitosan-coated
PLGA nanoparticles on temperature: 1—nanoparticles in bidistilled
water; 2—nanoparticles in 10% methanol solution
Fig. 8 The microcalorimetric study of chitosan-coated PLGA nano-
particles, immersed in buffers with different acidity, ranging from pH
2 to 8.2. The buffer molarity was 0.02 M Na
2
HPO
4
and 0.01 M citric
acid. The solid line—pH 2; the short dot line—pH 3.8; short dash dot
line—pH 5; dash line—pH 8.2
Calorimetric and spectrophotometric investigation of PLGA nanoparticles and their complex with DNA 341
123

In the case of interaction between non-coated NP and
DNA we haven not observed complex formation even
when the ratio was 24:1. The width of heat absorption
peaks were the same in both cases (Fig. 10), in contrast to
the mixture of DNA and chitosan-coated NP (see Fig. 9).
After the heat absorption peak, approximately in the tem-
perature interval from 40 to 100 °C, the values of heat
capacity are parallel to the temperature axis, what means
that there is no aggregation process between the destroyed
particles in this temperature interval. The aggregates
originate at 100–140 °C and at higher temperatures the
destruction process of aggregates to simple particles is
observed.
For more exact determination of DNA–particle inter-
action the spectrophotometric and ultracentrifugation
methods were used. Foreseeing the contribution of
chitosan-coated NP in spectrum, the spectra of pure DNA
with the spectrum of the same concentration DNA plus
chitosan-coated NP have been compared. We had the next
idea: if DNA interacts with NP so the DNA have to be
attached with NP and during sedimentation the complex
would fall away to bottom. The dependence of DNA
absorption on wave-length at various NP/[DNA] ratios is
given in Fig. 11. As it is evident from this figure at the
beginning when the nanoparticle concentration is zero or
pure DNA spectrum is present in the solution (curve 1). By
adding chitosan NP to DNA solution and measure the
sample after centrifugation shows that the value of
absorption is decreased which is caused by the reduction of
free DNA due to adsorption onto the particles’ surface. For
high chitosan–NP concentrations DNA is adsorbed com-
pletely onto the particles, whereas in the case of DNA
interaction with non-coated NP, the spectrophotometric
experiments did not reveal such interaction. Figure 12
shows the dependence of DNA-non-coated nanoparticle
solution absorption on the wavelength at the same
NP/[DNA] ratios.
To determine the amount of DNA which can be adsor-
bed on the surface of the NP the dependence of the number
of NP was plotted against the DNA mass unit. Figure 13
depicts the absorption at 260 nm (maximal value for each
sample) at various chitosan-coated NP/DNA (solid line)
and non-coated NP/DNA (dotted line) ratios. As it is seen
from the curve of DNA-chitosan nanoparticle complex
(solid line) when cNP/DNA ratios higher than 7, the whole
DNA is associated with chitosan-coated NP. When these
ration is \7, DNA molecules appear in solution which are
not able to connect with NP any more, because the particles
surface is already saturated with DNA molecules. At the
end (onset diagram) we have the value of absorption for
pure DNA (2 OD) in the solution. The optimal number of
chitosan-coated NP, which could complex with DNA,
Fig. 9 Dependence of the specific heat capacity of chitosan-coated
PLGA nanoparticles on temperature: 1—the calorimetric curve of
chitosan-coated nanoparticles; 2—the complex of chitosan-coated
nanoparticles with DNA at 1200:1 (w/w) nanoparticles/[DNA] ratio.
Solvent: bidistilled water, pH 5.0
Fig. 10 Dependence of the specific heat capacity of non-coated
PLGA nanoparticles on temperature: 1—the calorimetric curve of
non-coated nanoparticles; 2—the complex of non-coated nanoparti-
cles with DNA at nanoparticles/[DNA] ratio 24:1 (w/w)
Fig. 11 The dependence of DNA–chitosan-coated nanoparticle solu-
tion absorption on wavelength at different nanoparticles/[DNA]
ratios: 1—0; 2—1.685; 3—4.045; 4—6.726; 5—11.37
342 M. Khvedelidze et al.
123

could be extrapolated from those curves. Finally, we con-
clude that the optimal ratio of chitosan-coated NP to DNA
is 7:1 (w
Np
/w
DNA
).
Discussion
From the calorimetric results in Fig. 1we can conclude
that the coating of NP with chitosan did not exert an
influence on the behavior of the particles. The thermody-
namic parameters, in particular the profile of temperature
dependence of the specific heat absorption curves for non-
coated NP and chitosan-coated NP were the same. Also it is
clear that the calorimetric curves themselves have a
complex shape (Fig. 1) which is composed out of the heat
absorption peak area (23–35 °C), the sharp change at
T*130 °C, and the monotonic sections from 10–23 °C
and 35–130 °C. Such complicated nature of the calorimetric
curves indicates that the temperature has multifarious
influences on the NP and causes significant modification
of their structure. It should be mentioned that the influence
of temperature on NP starts right at the beginning of
the experiment, namely at 10 °C, where an increasing
heat absorption is observed which turns into a peak at
23–35 °C temperature interval. At the beginning of the
experiments, respectively, at the start of temperature
scanning the increase of particle specific heat absorption
might be induced by increasing the particle volume, as it
happens typically for solid bodies. We reach a such con-
clusion, owing to the calorimetry construction, because
the DASM-4A calorimeter is a device whose ampule is
represented by platinum thin capillary which when it is
entirely infilled, measures the sample heat effect for only
the half of the filled volume. Other construction calo-
rimeters (calorimeters, whose measuring ampules are
hermetically closed) measure the whole investigated
sample heat effect and are less sensitive to particle wid-
ening effect [91]. In other words, as we have mentioned
above, in our case the DASM-4A calorimeter measures
only part (half) of the suspension filled up in the capillary
and if the volume of the particle changes in this part of the
capillary, this change instantly effects the heat capacity of
this volume phase. Moreover, based on aforesaid it is clear
that in such calorimeters it becomes necessary to know the
partial volume for measuring the sample’s heat capacity,
which we have calculated above. Finally it can be con-
cluded, that at initial temperatures the particle volume
increase takes place, the particle swelling, which at
23–35 °C temperature interval finishes with particle
cracking. Because in this case the break in particle
existing bonds takes place, the heat absorption peak is
springing up. Obtained by turbidity measurements curve
(Fig. 2) should be related with volume increase of
spherical NP (in mentioned temperature interval) until it
will be destroyed, as a result of this ([30 °C) the trans-
parency of suspension is changed not considerably. We
would like to underline the circumstance, that while
increasing the temperature of suspension the small (not
large) increase of nanoparticles’ volume takes place,
which results the decrease of concentration of NP in
suspension (the number of particles in the unit of volume
will be less) and consequently the increase of turbidity
occurs. This experimental datum is explaining the increase
of specific heat capacity of NP in the above mentioned
temperature interval which takes place during calorimetric
experiments. On the other hand it should be mentioned
that the shape and also the size of the NP did not change
even for such high temperature as 150 °C, because cool-
ing them back to room temperature give us the homoge-
neous suspension which absorption spectrum is analogous
Fig. 12 The dependence of DNA-non-coated nanoparticle solution
absorption on the wavelength at different nanoparticles/[DNA] ratios:
1—0; 2—1.685; 3—4.045; 4—6.726; 5—11.37
Fig. 13 The dependence of DNA-nanoparticle solution absorption at
260 nm wavelength on nanoparticles/[DNA] ratio in the case of
chitosan-coated nanoparticles (solid line) and non-coated nanoparti-
cles (dotted line)
Calorimetric and spectrophotometric investigation of PLGA nanoparticles and their complex with DNA 343
123

to the spectrum of non-heated nanoparticle suspension.
This is supported by spectrophotometric data showing the
so-called Rayleigh scattering (the scattering intensity
I*k
-4
) which is the same for ‘‘native’’ and destroyed
particles (Fig. 3). Moreover, the spectrum of the destroyed
particles, obtained by heating up to 150 °C, did not differ
from the spectrum of the ‘‘native’’ particles or they are
just cracked by heating. This indicates that the particles
(polymer composition) themselves are coctostabile;
otherwise they would be destroyed entirely at high tem-
peratures. Therefore the optic spectroscopy, in particular
turbodimetric method, is not able to distinguish the initial
and temperature-induced changed NP from each other.
After particle cracking at 30 °C, from our point of view,
on the surface of cracked particles additional hydrophobic
chemical groups are exposed which avoid water molecules
and try to connect with the neighbouring particle surface
hydrophobic groups. It betokens that particles will
aggregate. From calorimetric curve we can see that after
the heat adsorption peak, at higher temperature the spe-
cific heat capacity curve is decreasing approximately from
50 to 120 °C area (Fig. 1), which is the typical case of
aggregates appearing. A further increasing of temperature
(above 120 °C) leads to a sharp increase of the heat
capacity curve, and we suppose that it is caused by
aggregate/conglomerate dissociation, perhaps to suspen-
sion of NP. The hydrophobic part of torn NP falls into the
contact with water and with further temperature increase
the amount of aggregates raises.
The herein described approaches to characterize PLGA
nanospheres may help the formulation scientists to become
aware of the complexity of this process and understand it
better. Also it should be mentioned that one of our purposes
was to study of possibility using chitosan NP for DNA
delivery, because they can complex anionic DNA on the
surface of the NP to obtain gene carriers. We suppose that
it does not matter would NP be closed or broken circuit,
DNA is able to fold over the surface of both type of
particle. Therefore the chitosan-coated PLGA NP with
PLA/PGA ratio 70:30 could be used for this purpose.
Experimental data of Fig. 4points out that particles
destruction does not occur instantly and this process is
stretched in time. The obtained thermodynamical results
point out that the nanoparticle destruction process is not
equilibrium. The particle destruction degree depends on the
heating rate, because there are different values for enthalpy
and specific heat capacity of NP at different heating rates.
The minimal heating rate of existing scanning calorimeter
is not enough for determining the tightness breaking
equilibrium temperature. Earlier it has been shown that
PLGA (50:50) NP, with inherent viscosity of 0.69 dL g
-1
coated with PVA, have glass transition temperature (T
g
)
onset 38 °C and endset 45 °C[91]. The heating speed in
those calorimetric experiments was 10 °C min
-1
. Because
of thermal gradients, the high heating rate caused the
transition temperature value increase. In other words, the
closer we can come to equilibrium the exacter the thermal
parameters can be measured.
This is also supported by the so-called annealing
experiments where NP first were heated only until 23 °C
(Fig. 5). The significant changes in the enthalpy and tran-
sition temperature (T
m
) from 30 to 25 °C in the reheating
curve show that if the particles are heated till the temper-
ature, which is required to start their internal transition, the
process proceeds spontaneously—no more energy is nee-
ded to destroy them up to the end. This result would be
absolutely understandable, if the particles would be
destroyed under the influence of temperature and they
could not restore their ‘‘native’’ structure. This experiment
show that the particles destroy equilibrium temperature
is less than 23 °C because after stopping temperature
scanning in calorimeter, during that little time while the
temperature of suspension was 23 °C the significant
decrease of nanoparticle stability takes place (Fig. 5).
The results of Fig. 6confirm once more that the particles
are unbroken and there is no inside solution leaking from
such particles by diffusion as it was indicated in literary data
[32,47,48]. Otherwise there must be the differences in
recording between freshly prepared nanoparticle and parti-
cle properties after some months at low temperature interval
(10–40 °C) in calorimetric curves where the heat absorption
peak is observed and time influences only on particle
aggregation process.
It is clear that the existence of hydrophobic forces in
creation and stability of nanoparticle emphasizes the high
profile. The NP destruction temperature depends on the
contact forces which originate during production. The
main part belongs to hydrophobic forces. However, their
strength depends on the solvent properties around the
particles. Changing (decrease) the polarity of the solvent
would have significant influence (diminution) on particle
transition temperature. In other words the stability of the
particles must depend on the extent of solvent’s polarity.
In Fig. 7we can see T
m
of the particles’ diminished right
away (from 30 up to 24 °C) when a 10% ethanol solution
is used. Moreover because of the solvent’s hydrophobicity
decrease there was no particles’ aggregation process
observed.
An important parameter for future applicability of NP-
based delivery systems in pharmaceutics is their stability
under relevant environmental conditions to avoid their
damage and prematurely drug release. Therefore, the
influence of temperature, pH and various salts on NP needs
to be investigated. At first, an unchanged T
m
(Fig. 8)
indicates the stability of the NP in all investigated pH-
values. The NP maintain their structure in deep acid (pH 2)
344 M. Khvedelidze et al.
123

as well as in alkaline (pH 8.2) conditions (heat absorption
peak at 30 °C, Fig. 8), i.e. the particles are not destructed
under these conditions. Moreover, the constant heat
capacity in the acid range for T[T
m
reflects the aug-
mented stability due to electrostatic repulsion, in contrast
to water and alkaline ranges. This is valid and expected for
chitosan-coated NP. For non-coated NP this would be
surprising because of the absence of any pH influence [91].
Therefore besides the particles are stable in buffer, in pH 2
(stomach pH) there is no aggregation process observed.
The experiments unambiguously show that in a wide pH
interval (2–8) the changes in transition temperature did not
take place. These results are important for two reasons:
such NP (PLA/PGA ratio in these PLGA NP is 70:30)
could be used in acidic surrounding (for instance, in
stomach) for drug transfer and the particles structure, sta-
bility and their other properties are less depended on either
the particles were in water (bidistilled or deionized) or the
suspension of particles were located in buffer (at least in
buffer with low molarity).
For determination the interaction between DNA and
NP calorimetric experiments have been carried out. For
non-coated NP no interaction with DNA was observed
spectrophotometrically, independently on the mixing ratio
(Fig. 10). This is not surprising, because of the negative
charge of both components. However, there is a difference
between non-coated NP and DNA-non-coated nanoparticle
complex calorimetric curves in the high temperature
regime. This means that negatively charged particles
aggregation temperature interval is rather moved towards
higher temperatures ([120 °C, Fig. 10) as in the interval of
30–110 °C NP aggregation did not occur.
When DNA was added to chitosan-coated NP suspen-
sion aggregates appeared in solution, what unambiguously
points to arising of complex between DNA and NP due to
electrostatic attraction between negatively charged phos-
phate groups of DNA molecule and positively charged
chitosan-coated NP. This complex springs up in such way
that the DNA wraps around such particles. The loading of
the particles limited only for small quantities of DNA or
better at high ratios of cNP/DNA (*1200:1). For smaller
ratios aggregates were observed in solution (Fig. 9).
In spite of such little amount of DNA the constriction
of calorimetric heat absorption peak confirms this
interaction.
The investigation at high cNP/DNA ratios was not
possible because of heterogeneity of solution. For this
reason it would be used the other alternative methods such
as UV spectrophotometry and ultracentrifugation. To
determine the binding ratio we used UV/Vis spectroscopy.
Formed complexes subsided and therefore the absorption
of DNA in the supernatant is decreased by the adsorbed
amount. In Fig. 11 the decrease of the absorption of free
DNA with increasing NP concentrations can be seen.
Plotting the dependence of the supernatant’s absorption for
different NP/[DNA] ratios (Fig. 13), allows to determine
the optimal quantity of DNA connected to the NP. Earlier it
was shown that the amount of DNA which could be
associated with NP is 1:50 (w/w), with cationic polymer is
from 1:0.4 to 1:6 (w/w) and in the case of lipid based
systems is from 1:2 to 1:6 (w/w) [1]. Our experimental
results show that the amount of DNA which could be
associated with chitosan-coated NP with 148 nm size is
1:0.14 (w/w), or 7 g of such particles are able to attach 1 g
of DNA.
It should be mentioned that the present method gives the
possibility to determine the NP interaction not only with
DNA, but also for other remedies. It allows determine the
minimum of a substance to be delivered, required to sat-
urate the NP.
Conclusions
Thus the carried out experiments show how necessary to
study the properties of PLGA NP by physical methods.
Experiments, which main purpose was to study the stability
of NP, reveal some significant properties. Coating the NP
with chitosan does not affect on their thermodynamic and
optic behavior. It has shown that particle thermal stability
depends on solvent polarity degree—as more polar solvent
more is thermo stable of NP (adding the ethyl acetate
decrease the stability of NP). It has been shown that the
tightness breaking temperature depends on particle sus-
pension heating rate. The minimal heating rate of existing
scanning microcalorimeters is not enough for determining
the tightness breaking equilibrium temperature. Study of
NP stability reveals that at about 20 °C their internal vol-
ume tightness is damaged. Therefore the interior sub-
stances leakage in outer space would take place. From
obtained results it has shown that the NP wholeness does
not lose: the shape and size is survived even in such tem-
perature as 150 °C, though the leak tightness is damaged. It
has shown that the NP maintain their structure wholeness
in a wide pH acidity interval (2.0–8.0) and not only in
bidistilled or deionized water (at least in buffer with low
molarity), what is important for drug transfer orally without
nanoparticle tightness damage.
It was determined the amount of DNA which can be
adsorbed on the surface of the NP which turned out that
the optimal ratio of chitosan-coated NP to DNA is 7:1
(W
Np
/W
DNA
). It should be mentioned that the present
method gives the possibility to determine the NP interac-
tion not only for DNA, but also for other remedies. It
allows determine the minimum of a substance to be
delivered, required to saturate the NP.
Calorimetric and spectrophotometric investigation of PLGA nanoparticles and their complex with DNA 345
123

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