Nanomaterials 10 01042 v2
Nanomaterials 10 01042 v2
Nanomaterials 10 01042 v2
Article
Dispersibility and Size Control of Silver
Nanoparticles with Anti-Algal Potential Based on
Coupling Effects of Polyvinylpyrrolidone and
Sodium Tripolyphosphate
Mingshuai Wang, Haibo Li *, Yinghua Li *, Fan Mo, Zhe Li, Rui Chai and Hongxuan Wang
School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China;
wms1995@126.com (M.W.); moliangming4312975@163.com (F.M.); lizhe1824@163.com (Z.L.);
chairuicici@126.com (R.C.); 18245706557@163.com (H.W.)
* Correspondence: lihaibo@mail.neu.edu.cn (H.L.); liyinghua@mail.neu.edu.cn (Y.L.)
Received: 10 May 2020; Accepted: 27 May 2020; Published: 29 May 2020
Abstract: In nearly all the cases of biotoxicity studies of silver nanoparticles (AgNPs), AgNPs used
often have general dispersibility and wide size distribution, which may inevitably generate imprecise
results. Herein, a kind of synthesis method by coupling effects of polyvinylpyrrolidone (PVP) and
sodium tripolyphosphate (STPP) was proposed, in order to prepare AgNPs with better dispersibility
and a stable size. Based on this, the preparation mechanism of AgNPs and the potential anti-algae
toxicity were analyzed. UV-vis analysis showed that the particle size distribution of AgNPs prepared
by co-protective agents was more uniform. X-ray diffraction (XRD), field emission scanning electron
microscopy (FE-SEM), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX)
were used to confirm that the obtained nano silver was of a high purity and stable size (~30 nm
in diameter). Zeta potential and Fourier transform infrared spectroscopy (FTIR) analysis results
indicated the synthesis mechanism of AgNPs by co-protective agents, more precisely, PVP limited
the polynegative effect and prevented the linear induction of P3 O10 5− produced by STPP during the
growth of silver nuclei. Subsequently, Chlorella and Scenedesmus obliquus were utilized to test the
toxicity of AgNPs, confirming that AgNPs synthesized through co-protective agents have potential
inhibitory ability on algae, but not severe. This study provides a basic theory for the induction of
synthetic AgNPs by various factors in the natural environment and a scientific reference for the
environmental risk assessment.
1. Introduction
Over the past decades, silver nanoparticles (AgNPs) have been widely applied for their unique
dimensional structures and performance superiorities, such as antibacterial [1,2], antifungal [3,4],
anti-algal [5,6], catalytic [7,8], and photoelectric properties [9,10]. Many investigations into the
biological and chemical synthesis methods of AgNPs with multiple shapes and sizes have been
reported [11–15]. Although these previous results were expansive, data to explain the growth process
are still scare. Such an explanation requires the exploration and perfection of the synthetic mechanism.
In general, there are two protective mechanisms for maintaining the stability of nanoparticles
during chemical synthesis, namely the electrostatic repulsion of small molecule protectants [16]
and the steric hindrance of macromolecular polymers [17]. When the pH of an aqueous solution
is higher than the zero-potential pH (pHPZC ) of nano silver, its surface without coating adsorbs
some negatively charged groups (e.g., hydroxide ions and oxygen-containing groups) and maintains
stability via electrostatic repulsion [18]. Nano silver that is stabilized by steric hindrance has a
stronger anti-electrolyte interference ability and can exist stably under relatively high ionic-strength
conditions [19]. However, few studies [20,21] have combined two protective mechanisms to explore
their common mechanism for the synthesis of AgNPs.
Furthermore, in the biotoxicity experiments of AgNPs, the studies show that the most obvious
toxic effect of AgNPs on algae is to inhibit growth [22]. It may also adsorb and aggregate on the surface
of algae cells, which in turn leads to the destruction of the cell membrane or skeleton structure and
to the damage of organelle functions, thus inhibiting the material metabolism and photosynthesis of
algal cells [23]. However, most of them used AgNPs with general dispersibility and wide particle size
distribution for experiments, which would affect the evaluation of their biological toxicity and make
them difficult to be convinced. The dispersibility of AgNPs in the water environment may have a
difference in the toxicity of biomolecules, and this difference has not been confirmed.
Therefore, a hypothesis would be proposed that sodium tripolyphosphate (STPP) was used as a
small molecule protectant and polyvinylpyrrolidone (PVP) was used as a macromolecular polymer
to investigate whether coupling mechanism would occur during the synthesis of AgNPs. The steric
hindrance effect of PVP and the electrostatic repulsion effect of STPP may synergistically promote
the dispersibility and stability of AgNPs or affect the entire redox process. In this present study,
PVP and STPP were used together as a protective agent for synthesizing AgNPs, with silver nitrate
(AgNO3 ) and glucose as the precursor of silver and the reducing agent, respectively. The obtained
nanoparticles were characterized by ultraviolet-visible (UV-vis) spectroscopy, X-ray diffraction (XRD),
field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), zeta
potential, and Fourier transform infrared spectroscopy (FTIR) analyses, as a means of explaining the
coupling mechanism of particle size growth and stability during the synthesis process. Finally, Chlorella
and Scenedesmus obliquus were selected to compare the toxicity of nano silver, and we examined the
inhibition potential of AgNPs with different dispersibility to algae.
2.1. Materials
Silver nitrate (AgNO3 ) (99.8%) and sodium tripolyphosphate (Na5 P3 O10 ), which were both
analytical reagents (ARs), were supplied by Sinopharm Chemical Reagent Co. Ltd. (Shenyang, China)
Polyvinylpyrrolidone (PVP) (K30) was obtained from Macklin (Shanghai, China). Glucose and sodium
hydroxide (Zhiyuan Chemical Reagent Co. Ltd., Tianjin, China) were both reagent-grade, as-received
materials. Deionized water was used as a solvent throughout the experiment.
provide a temperature control of 25 ◦ C and 2000 lux light intensity. The growth and cultivation status
of algae are available in the Figures S1 and S2.
where V is the chlorophyll extraction volume (mL), and W is the biomass weight of algal (g).
Figure 1. (a) Ultraviolet-visible (UV-vis) spectra of individual protective agents versus combined
protective agents. (b) UV-vis spectra of silver nanoparticles (AgNPs) with different initial
Figure
Figure 1. (a)
1. (a) Ultraviolet-visible
concentrationsUltraviolet-visible (UV-vis)
(UV-vis) spectra
of AgNO3. Polyvinylpyrrolidone of
spectraisof individual
individual
indicated protective
protective
by p-, agents
agents
and sodium versus combined
versus combined
tripolyphosphate by
protective
protective
s-. agents.(b)(b)
agents. UV-vis
UV-vis spectra
spectra ofnanoparticles
of silver silver nanoparticles (AgNPs)
(AgNPs) with with
different different
initial initial
concentrations
concentrations
of of AgNO3. Polyvinylpyrrolidone
AgNO3 . Polyvinylpyrrolidone is indicated by is p-,indicated by p-,
and sodium and sodium tripolyphosphate
tripolyphosphate by s-. by
s-.
3.1.2. Effects of Temperature and Silver Nitrate Concentration
3.1.2. Effects of Temperature and Silver Nitrate Concentration
Figure
3.1.2.Figure
Effects2a 2a displays
ofdisplays
Temperature the UV-vis
and Silverabsorption spectra
Nitratespectra of AgNPs prepared at different temperatures
Concentration
the UV-vis absorption of AgNPs prepared at different temperatures
(40, 60, 80, and 100◦ °C), with a reaction time of 20 min. At first, as the temperature rose from 40 to◦80
(40, 60, 80, and
Figure 2a 100 C), with UV-vis
displays a reaction time of 20spectra
min. Atoffirst, as the temperature rose from 40 to 80 C,
°C, the absorption peak the absorption
at 400 nm gradually became stronger;AgNPs prepared
hence, at different
more silver ions weretemperatures
converted
the
(40, absorption
60, 80, and peak
100 at
°C), 400
with nm
a gradually
reaction timebecame
of 20 stronger;
min. At hence,
first, as more
the silver
temperature ions were
rose converted
from 40 to 80
into nano silver. In addition, such a raise in temperature would have accelerated the movement of
into
°C, nano
the silver. Inpeak
absorption addition,
at 400 such
nm a raise inbecame
gradually temperature would
stronger; have
hence, accelerated
more silver ionsthe movement
were convertedof
ions, thereby making electrostatic forces active, which would have enhanced the stability. However,
ions,
into thereby
nano making
silver. electrostatic
Intoaddition, suchforcesraiseactive,
ahave which would have
in temperature enhanced the stability. However,
a further increase 100
◦ C°C would led to a decreasewould
in thehave accelerated
maximum the movement
absorbance value. Thisof
aions,
further increase to 100 would have led to a decrease in the maximum absorbance value. This might
mightthereby
indicate making
that theelectrostatic forces active,
higher temperatures which would
accelerated the ratehave enhanced
of AgNPs the stability.
synthesis, However,
thus causing the
indicate
afaster
further that the higher
increase to 100 temperatures
°C would accelerated
have led to a the rate of in
decrease AgNPs
the synthesis,absorbance
maximum thus causing the faster
value. This
nucleation growth. When the particle size of nano silver became larger, its dispersibility
nucleation
might indicategrowth. When
that the the particle
higher size ofaccelerated
temperatures nano silverthebecame
rate larger, its synthesis,
dispersibility thusdeteriorated,
deteriorated, resulting in agglomeration [29]. Therefore, an of AgNPs
equilibrium point between causing the
a good
resulting
faster in agglomeration
nucleation [29]. Therefore, an equilibrium point between a good synthesis rate and a
synthesis rate andgrowth.
a uniform When the size
particle particle size of nano
distribution silver at
were found became larger,temperature
the reaction ◦
its dispersibility
of 80
uniform
deteriorated,particle size distribution
resulting were found
in agglomeration [29].atthe
the reactionantemperature
Therefore, ofpoint
80 C.between
An even-higher
°C. An even-higher temperature might destroy electrostatic equilibrium
repulsion between AgNPs. a good
temperature might destroy the electrostatic repulsion between AgNPs.
synthesis rate and a uniform particle size distribution were found at the reaction temperature of 80
°C. An even-higher temperature might destroy the electrostatic repulsion between AgNPs.
(a) (b)
Figure 2. (a)
Figure 2. (a)
(a) Ultraviolet-visible
Ultraviolet-visible (UV-vis)
(UV-vis) spectra
spectra of
of ps-AgNPs
ps-AgNPs synthesized at(b)
synthesized at different
different temperatures,
temperatures,
using
using 0.02 mol/L AgNO3. (b) UV-vis spectra of ps-AgNPs synthesized with different initial
0.02 mol/L AgNO 3 . (b) UV-vis spectra of ps-AgNPs synthesized with different initial
concentrations
Figure of
of AgNO
AgNO33..
2. (a) Ultraviolet-visible
concentrations (UV-vis) spectra of ps-AgNPs synthesized at different temperatures,
using 0.02 mol/L AgNO3. (b) UV-vis spectra of ps-AgNPs synthesized with different initial
The effect of AgNO3 at
concentrations of AgNO 3.
different initial concentrations in the reaction is exhibited in Figure 2b.
The highest absorption peak position of the prepared colloids was around 400 nm. It is generally
Nanomaterials 2020, 10, 1042 6 of 15
believed that the formation of AgNPs includes two processes, nucleation and crystal growth [30].
These two processes existed simultaneously in the entire reaction of adding AgNO3 solution, but
nucleation dominated during the early stage, and nuclear growth dominated later. As the concentration
of silver ions in the colloid increased (0.01, 0.02, and 0.03 mol/L), more nuclei were formed during the
early stage, and sufficient silver was supplied to the nuclei for growth. Therefore, the concentration of
nano silver increased, and the absorption peak position was higher. By further increasing the AgNO3
concentration, the maximum absorbance value began to show a downward trend. We infer that the
scope and ability of the co-protective agents were limited. Increasing the AgNO3 concentration would
have resulted in frequent collisions between grains, thereby leading to the aggregation and deposition
of nanoparticles. Furthermore, we also reported the pH of the solution before and after the AgNPs
were synthesized in Table 1.
Table 1. The pH of the solution before and after the ps-AgNPs were synthesized with different initial
concentrations of AgNO3 .
The Concentration of
0.01 0.02 0.03 0.04 0.05
AgNO3 (mol/L)
pH before synthesis 10.58 10.35 10.17 10.22 10.14
pH after synthesis 5.82 5.44 4.74 4.67 4.83
The pH is an important factor affecting the ionization equilibrium and electrode potential of the
reaction system. Before the synthesis of AgNPs, the overall solution showed alkalinity due to the
ionization of sodium hydroxide. Moreover, as the concentration of AgNO3 increased, the alkalinity of
the reaction system decreased. It was because the hydrolysis of AgNO3 showed weak acidity, which
neutralized part of hydroxyl ion (OH− ). After the synthesis, the reaction solution showed acidity,
indicating that OH− participated in reduction of Ag+ to form Ag by glucose and generated acidic
substances. The addition of alkali was conducive to higher reducing ability; however, it had an adverse
effect on particle agglomeration [31]. According to the acid–base change of the solution, we speculated
that the following reaction process might occur during the synthesis of ps-AgNPs:
Ag2 O + CH2 OH (CHOH)4 CHO + 2PVP → CH2OH (CHOH)4 COOH + 2Ag(PVP)↓ (3)
Figure 4.4.Images
Figure Imagesand histograms
and of polyvinylpyrrolidone
histograms (PVP)(PVP)
of polyvinylpyrrolidone plus sodium tripolyphosphate
plus sodium (STPP)
tripolyphosphate
silver
(STPP) nanoparticles (ps-AgNPs)
silver nanoparticles with different
(ps-AgNPs) AgNO3 concentrations.
with different FE-SEM: FE-SEM:
AgNO3 concentrations. field emission
field scanning
emission
electron
scanning
Figure 4.microscopy;
electron TEM: transmission
Images microscopy;
and electron microscopy.
TEM:oftransmission
histograms electron microscopy.
polyvinylpyrrolidone (PVP) plus sodium tripolyphosphate
(STPP) silver nanoparticles (ps-AgNPs) with different AgNO3 concentrations. FE-SEM: field emission
scanning electron microscopy; TEM: transmission electron microscopy.
Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 15
Imagesofof(a,d)
Figure5.5.Images
Figure (a,d)polyvinylpyrrolidone
polyvinylpyrrolidone silver
silver nanoparticles
nanoparticles(p-AgNPs)
(p-AgNPs) andand(b,e) sodium
(b,e) sodium
tripolyphosphatesilver
tripolyphosphate silver nanoparticles
nanoparticles (s-AgNPs)
(s-AgNPs) at at 0.02
0.02 mol/L
mol/LAgNO
AgNO 3 , 3,and
and(c,f) images
(c,f) imagesof of
polyvinylpyrrolidone (PVP) plus sodium tripolyphosphate (STPP) silver nanoparticles
polyvinylpyrrolidone (PVP) plus sodium tripolyphosphate (STPP) silver nanoparticles (ps-AgNPs) (ps-AgNPs) at
0.01 mol/L AgNO
at 0.01 mol/L AgNO in low magnification. FE-SEM: field emission scanning electron microscopy;
3 3 in low magnification. FE-SEM: field emission scanning electron microscopy; TEM:
transmission electron microscopy.
TEM: transmission electron microscopy.
EDX spectra (Figure 6) analysis confirmed the presence of elemental silver by sharp signals around
EDXwhich
3 keV, spectra
is a(Figure 6) analysis
typical range confirmed
of metallic the presence
nano crystallites of elemental
optical absorptionsilver by sharp
band [33]. signals
The peak
around 3 keV,
observed which
at 1.75 keV is a typical
belonged to range of metallic
Si, which nano
was formed crystallites
from opticalAabsorption
the SEM carrier. band
spurious peak [33]. The
between
peak observed at 1.75 keV belonged to Si, which was formed from the SEM carrier.
0 and 2 keV that corresponds to C was derived from the raw materials, and C was coated onto the A spurious peak
between
surface0ofand 2 keV
AgNPs in that corresponds
the form to Cagents.
of protective was derived
No otherfrom the raw
obvious materials,
impurity peaksand C detected
were was coated
onto the surface
throughout the of AgNPsrange
scanning in theof form of protective
binding agents.
energies. Thus, Noapparent
it was other obvious
that theimpurity peaks
EDX analysis were
also
detected
providedthroughout
evidence ofthe scanningAgNPs.
high-purity range of binding energies. Thus, it was apparent that the EDX
analysis also provided evidence of high-purity AgNPs.
3.2.3. Zeta Potential
As shown in Figure 7, zeta potential was tested for different concentrations and types of nano silver,
to determine their stability. Overall, the synthesized AgNPs were negatively charged, which kept the
nanoparticles in the colloidal suspension. With increased nano silver concentration, the zeta potential of
p-AgNPs decreased and that of the s-AgNPs increased. PVP is primarily used to stabilize nanoparticles
by providing steric resistance, rather than by electrostatic repulsion [34]. As a result, p-AgNPs is less
electronegative. The tripolyphosphate (P3 O10 5− ) produced by STPP acted as a polyanion that interacts
with cationic substances by electrostatic force [35]. As its concentration increased, this electrostatic
repulsion became more pronounced. The zeta potential of ps-AgNPs was lower at lower concentrations,
thus showing an insignificant electrostatic repulsion. When the concentration was increased, the zeta
Nanomaterials 2020, 10, 1042 9 of 15
potential was centered in contrast with the other two AgNPs, thereby indicating that PVP limited the
polynegative effect of P3 O10 5− . Combining the microscopy images of the three AgNPs, we observed
that the aggregation state of s-AgNPs was the worst, while p-AgNPs and ps-AgNPs had better
dispersibility. It was concluded that the steric hindrance of PVP played a leading role in maintaining
Figure 6. Energy dispersive X-ray (EDX) spectra of polyvinylpyrrolidone (PVP) plus sodium
the stability2020,
Nanomaterials of nanoparticles.
10, x FOR PEER REVIEW 9 of 15
tripolyphosphate (STPP) silver nanoparticles (ps-AgNPs) at 0.02 mol/L AgNO3.
Figure 7. Zeta potentials of the three silver nanoparticles (AgNPs) at different concentrations.
Polyvinylpyrrolidone is indicated by p-, and sodium tripolyphosphate by s-.
this reaction rarely occurred, and the reduction of Ag+ by glucose was merely carried out. On the
other hand, the metal carbonyl CO stretching at 1971.94 cm−1 indicates that the aldehyde group (CHO)
of glucose had been broken and formed the metal carbonyl with the transition metal Ag. Similarly,
this reaction did not occur under the coupling of PVP and STPP. It is worth noting that the peaks at
Nanomaterials 2020, 10, 1042 10 of 15
2909.21 and 1971.94 cm−1 were weak, indicating a small reaction intensity. Additionally, the distorted
vibration of the functional group NH2 at 803.24 cm−1 also supported the aforementioned conjecture.
1971.94 cm−1 were weak, indicating a small reaction intensity. Additionally, the distorted vibration of
the functional group NH2 at 803.24 cm−1 also supported the aforementioned conjecture.
Figure
Figure 8. 8. Fouriertransform
Fourier transform infrared spectroscopy
infrared (FTIR)(FTIR)
spectroscopy spectra of three silver
spectra nanoparticles
of three (AgNPs).
silver nanoparticles
Polyvinylpyrrolidone is indicated by p-, and sodium tripolyphosphate by s-.
(AgNPs). Polyvinylpyrrolidone is indicated by p-, and sodium tripolyphosphate by s-.
3.3. Formation Mechanism
3.3. Formation Mechanism
The formation of nanoparticles mainly includes three stages: reduction of ions, grouping of
The formation
nanoparticles, andof nanoparticles
subsequent growthmainly includes Moreover,
of nanoparticles. three stages: reduction
the reduction of ions,
process grouping
of metal ions of
nanoparticles,
is affected by and subsequent
multiple factors, growth
includingofthenanoparticles. Moreover,
reaction conditions the reduction
(e.g., temperature, pH, process of metal
and reaction
ions time)
is affected by multiple
of the mixture system,factors, including
electrochemical the reaction
changes of metal conditions (e.g., temperature,
ions, and properties pH, and
of the protective
agents
reaction [37]. of the mixture system, electrochemical changes of metal ions, and properties of the
time)
The
protective agentsfollowing
[37].are formation mechanisms for the synthesis of ps-AgNPs with a stable particle size.
Firstly, during the reduction of Ag+ to silver, STPP as a protective agent can dissociate into P3 O10 5− ,
The following are formation mechanisms for the synthesis of ps-AgNPs with a stable particle
which would have combined with Ag+ to form silver triphosphate (Ag5 P3 O10 ). As a consequence of
size. Firstly, during the reduction of Ag+ to silver, STPP as a protective agent can dissociate into
the limited solubility of Ag5 P3 O10 in the solution, the concentration of Ag+ is maintained at a stable
P3O10level,
5− , which would have combined with Ag to form silver triphosphate (Ag5P3O10). As a
+
which prevents the uneven particle size distribution caused by an excessive reduction rate and
consequence
stabilizes theof particle
the limited
size. Atsolubility
the same of Agthe
time, 5P3O10 in the solution, the concentration of Ag+ is
addition of PVP also promotes the stabilization of
maintained
the colloid at and
a stable level,
prevents which prevents
agglomeration the uneven
among particles. Thisparticle
is mainlysize distribution
because causedinby an
the polar groups
excessive reduction rate and stabilizes the particle size. At the same time, the
its structural unit contain N and O atoms, both of which have lone pair electrons such that they addition of PVP
can also
promotes
provide the stabilization
electron of the
clouds for empty colloid and orbitals
sp hybrid prevents +
of agglomeration among particles.
Ag to form coordination This
bonds [38]. is is,
That mainly
the sp orbits are hybridized to form a silver ion complex. Moreover, the linear
because the polar groups in its structural unit contain N and O atoms, both of which have lone pair PVP can continue to
adsorbsuch
electrons on the surface
that theyofcan
colloidal
provideparticles after the
electron reduction
clouds of silver.
for empty spThe protective
hybrid layerofformed
orbitals Ag+ to at form
the solid–liquid interface can not only effectively reduce the surface
coordination bonds [38]. That is, the sp orbits are hybridized to form a silver 5− energy, but also uniformly disperse
ion complex. Moreover,
AgNPs in the liquid-phase system [39]. Meanwhile, the linear induction of P3 O10 during the growth
the linear PVP can continue to adsorb on the surface of colloidal particles after the reduction of silver.
of silver nuclei is prevented, and the agglomeration problem is alleviated. In addition, the coupling of
The protective layer formed at the solid–liquid interface can not only effectively reduce the surface
PVP and STPP as protective agents may also limit the progress of other redox reactions.
Nevertheless, we observed that, regardless of the type of protective agent that was used to prepare
the nano silver, different degrees of aggregation would occur after a period of time. This indicates
that the dispersion effect of the protective agent would weaken with time. As a result of the high
specific surface area and surface energy, the nano silver enters a thermodynamically unstable state,
progress of other redox reactions.
Nevertheless, we observed that, regardless of the type of protective agent that was used to
Nevertheless, we observed that, regardless of the type of protective agent that was used to
prepare the nano silver, different degrees of aggregation would occur after a period of time. This
prepare the nano silver, different degrees of aggregation would occur after a period of time. This
indicates that the dispersion effect of the protective agent would weaken with time. As a result of the
indicates that the dispersion effect of the protective agent would weaken with time. As a result of the
high specific surface area and surface energy, the nano silver enters a thermodynamically unstable
high specific
Nanomaterials
surface
state, and 2020,
the 10, area and
particles
1042
surface
easily energy,
fuse with eachtheother,
nanoinsilver enters
order a thermodynamically
to reunite unstable
[40]. Furthermore,
11 ofthe
15
state,
agglomeration force can still be opened by ultrasonic dispersion. The synthesis process of the ps-the
and the particles easily fuse with each other, in order to reunite [40]. Furthermore,
agglomeration force can
AgNPs is illustrated still be9.opened by ultrasonic dispersion. The synthesis process of the ps-
in Figure
AgNPs is illustrated in Figurewith
and the particles easily fuse 9. each other, in order to reunite [40]. Furthermore, the agglomeration
force can still be opened by ultrasonic dispersion. The synthesis process of the ps-AgNPs is illustrated
in Figure 9.
Figure 10. Reduction in total chlorophyll in algae at varying concentrations and types of silver
nanoparticles (AgNPs). (a) Chlorella. (b) Scenedesmus obliquus. Polyvinylpyrrolidone is indicated by p-,
and sodium tripolyphosphate by s-.
The total chlorophyll content in both the algal species (Chlorella and Scenedesmus obliquus) exhibited
a concentration-dependent decrease following exposure to the three AgNPs at different concentrations
(0.1, 0.3, and 0.5 mmol/L). The percentage of the total chlorophyll reduction increased with exposure to
the AgNPs. On the third day of Scenedesmus obliquus cultivation, the chlorophyll content decreased
significantly, by 19.0%, 21.6%, and 25.4%, in response to the applied ps-AgNPs at concentrations of
0.1, 0.3, and 0.5 mmol/L. Similarly, the reduction in the chlorophyll content was more pronounced,
Nanomaterials 2020, 10, 1042 12 of 15
exhibiting declines of approximately 36.2%, 43.1%, and 44.8%, at the corresponding doses of ps-AgNPs.
The decreased chlorophyll content that was caused by the silver toxicity, which was more evident in
Chlorella than in Scenedesmus obliquus, thus indicating a greater sensitivity of Chlorella against nano
silver toxicity. Additionally, the chlorophyll content of s-AgNPs decreased the least; that is, the effect
of s-AgNPs on photosynthetic pigments of algae was the weakest. Therefore, the s-AgNPs presented
a lower algae resistance than the other two AgNPs. Nano silver can be introduced to accumulate
slightly due to the presence of electrolytes in the BG11 medium [41]. As a result, the s-AgNPs, which
carried the most negative charges, were even less dispersed. The dispersion state of nano silver has
an important impact on its bioavailability and environmental behavior. It is generally believed that
nanoparticles can enter cells through holes in the cell wall [42]. Therefore, the severely agglomerated
s-AgNPs showed a poor anti-algal toxicity. At the same time, nanomaterials are more stable through
electrostatic repulsion and steric hindrance [43], which enhance the inhibition effect of the ps-AgNPs
on the algae photosynthetic pigment stability, thereby showing a better algae-suppression effect.
4. Conclusions
In summary, we used a new approach for the synthesis of AgNPs with a stable size and better
dispersibility, by combining PVP and STPP, and then we investigated the coupling mechanism and
tested its anti-algal potential. During the synthesis process, the AgNPs prepared by the synergistic
protection of PVP and STPP displayed a superior state. The structures analysis demonstrated that
the synthesized pure AgNPs was of a stable sized (~30 nm in diameter). The coupling mechanism
of the co-protective agents was proposed from the perspective of colloidal interface and potential.
The low-solubility Ag5 P3 O10 formed from STPP and the steric hindrance of PVP jointly limited the
reduction rate of Ag+ , preventing the uneven particle size distribution caused by the excessive reduction
rate and stabilizing the particle size. Meanwhile, PVP limited the polynegative effect of P3 O10 5− and
prevented the linear induction of P3 O10 5− during the growth of silver nuclei. Ultimately, the AgNPs
synthesized with co-protective agents exhibited the potential inhibitory ability in algal-resistance
experiments against Chlorella and Scenedesmus obliquus. This study provides a basic theory for the
synthesis of AgNPs induced by various factors in the natural environment and a scientific reference
for the risk assessment of water environment. For future studies, the co-effects of multiple protection
mechanisms on prepared materials can be further explored, to investigate whether the anti-algal
potential of nano silver is stable and durable.
References
1. Chen, S.Y.; Huang, M.T.; Pender, S.L.F.; Ruslin, M.; Chou, H.H.; Qu, K.L. The application of silver
nano-particles on developing potential treatment for chronic rhinosinusitis: Antibacterial action and
cytotoxicity effect on human nasal epithelial cell model. Mater. Sci. Eng. C 2017, 80, 624–630. [CrossRef]
[PubMed]
2. Gurunathan, S.; Han, J.W.; Kwon, D.N.; Kim, J.H. Enhanced antibacterial and anti-biofilm activities of silver
nanoparticles against Gram-negative and Gram-positive bacteria. Nanoscale Res. Lett. 2014, 9, 373–389.
[CrossRef] [PubMed]
3. Ogar, A.; Tylko, G.; Turnau, K. Antifungal properties of silver nanoparticles against indoor mould growth.
Sci. Total Environ. 2015, 521, 305–314. [CrossRef] [PubMed]
4. Xue, B.; He, D.; Gao, S.; Wang, D.; Yokoyama, K.; Wang, L. Biosynthesis of silver nanoparticles by the fungus
Arthroderma Fulvum and its antifungal activity against genera of Candida, Aspergillus and Fusarium. Int. J.
Nanomed. 2016, 11, 1899–1906.
5. Dash, A.; Singh, A.P.; Chaudhary, B.R.; Singh, S.K.; Dash, D. Effect of silver nanoparticles on growth of
eukaryotic green algae. Nano-Micro Lett. 2012, 4, 158–165. [CrossRef]
6. Kumari, R.; Barsainya, M.; Singh, D.P. Biogenic synthesis of silver nanoparticle by using secondary metabolites
from Pseudomonas aeruginosa DM1 and its anti-algal effect on Chlorella vulgaris and Chlorella pyrenoidosa.
Environ. Sci. Pollut. Res. Int. 2017, 24, 4645–4654. [CrossRef]
7. Zhang, P.; Shao, C.; Zhang, Z.; Zhang, M.; Mu, J.; Guo, Z.; Liu, Y. In situ assembly of well-dispersed Ag
nanoparticles (AgNPs) on electrospun carbon nanofibers (CNFs) for catalytic reduction of 4-nitrophenol.
Nanoscale 2011, 3, 3357–3363. [CrossRef]
8. Li, X.; Wang, J.; Zhang, Y.; Li, M.; Liu, J. Surfactantless synthesis and the surface-enhanced Raman spectra
and catalytic activity of differently shaped silver nanomaterials. Eur. J. Inorg. Chem. 2010, 12, 1806–1812.
[CrossRef]
9. Duan, C.; Wang, H.; Ou, X.; Li, F.; Zhang, X. Efficient visible light photocatalyst fabricated by depositing
plasmonic Ag nanoparticles on conductive polymer-protected Si nanowire arrays for photoelectrochemical
hydrogen generation. ACS Appl. Mater. Inter. 2014, 6, 9742–9750. [CrossRef]
10. Yu, A.; Wang, Q.; Wang, J.; Chang, C. Rapid synthesis of colloidal silver triangular nanoprisms and their
promotion of TiO2 photocatalysis on methylene blue under visible light. Catal. Commun. 2017, 90, 75–78.
[CrossRef]
11. Akaighe, N.; Maccuspie, R.I.; Navarro, D.A.; Aga, D.S.; Banerjee, S.; Sohn, M.; Sharma, V.K. Humic
acid-induced silver nanoparticle formation under environmentally relevant conditions. Environ. Sci. Technol.
2011, 45, 3895–3901. [CrossRef] [PubMed]
12. Sun, Y.; Xia, Y. Shape-controlled synthesis of gold and silver nanoparticles. Science 2002, 298, 2176–2179.
[CrossRef] [PubMed]
13. Shi, Y.; Lv, L.; Wang, H. A facile approach to synthesize silver nanorods capped with sodium tripolyphosphate.
Mater. Lett. 2009, 63, 2698–2700. [CrossRef]
14. Sangaonkar, G.M.; Pawar, K.D. Garcinia indica mediated biogenic synthesis of silver nanoparticles with
antibacterial and antioxidant activities. Colloid. Surf. B 2018, 164, 210–217. [CrossRef]
15. Jeon, S.H.; Xu, P.; Zhang, B.; Mack, N.H.; Tsai, H.; Chiang, L.Y.; Wang, H.L. Polymer-assisted preparation of
metal nanoparticles with controlled size and morphology. J. Mater. Chem. 2011, 21, 2550–2554. [CrossRef]
16. Li, X.; Lenhart, J.J.; Walker, H.W. Dissolution-accompanied aggregation kinetics of silver nanoparticles.
Langmuir 2010, 26, 16690–16698. [CrossRef]
17. Huynh, K.A.; Chen, K.L. Aggregation kinetics of citrate and polyvinylpyrrolidone coated silver nanoparticles
in monovalent and divalent electrolyte solutions. Environ. Sci. Technol. 2011, 45, 5564–5571. [CrossRef]
18. Levard, C.; Hotze, E.M.; Lowry, G.V.; Brown, G.E., Jr. Environmental transformations of silver nanoparticles:
Impact on stability and toxicity. Environ. Sci. Technol. 2012, 46, 6900–6914. [CrossRef]
19. Tejamaya, M.; Romer, I.; Merrifield, R.C.; Lead, J.R. Stability of citrate, PVP, and PEG coated silver
nanoparticles in ecotoxicology media. Environ. Sci. Technol. 2012, 46, 7011–7017. [CrossRef]
20. Aiken, G.R.; Hsu-Kim, H.; Ryan, J.N. Influence of dissolved organic matter on the environmental fate of
metals, nanoparticles, and colloids. Environ. Sci. Technol. 2011, 45, 3196–3201. [CrossRef]
Nanomaterials 2020, 10, 1042 14 of 15
21. Stankus, D.P.; Lohse, S.E.; Hutchison, J.E.; Nason, J.A. Interactions between natural organic matter and gold
nanoparticles stabilized with different organic capping agents. Environ. Sci. Technol. 2011, 45, 3238–3244.
[CrossRef] [PubMed]
22. He, D.; Dorantes-Aranda, J.J.; Waite, T.D. Silver nanoparticle-algae interactions: Oxidative dissolution,
reactive oxygen species generation and synergistic toxic effects. Environ. Sci. Technol. 2012, 46, 8731–8738.
[CrossRef] [PubMed]
23. David, D.; Oukarroum, A. Silver nanoparticles toxicity effect on photosystem II photochemistry of the green
alga Chlamydomonas Reinhardtii treated in light and dark conditions. Toxicol. Environ. Chem. 2012, 94,
1536–1546.
24. Sartory, D.P.; Grobbelaar, J.U. Extraction of chlorophyll a from freshwater phytoplankton for
spectrophotometric analysis. Hydrobiologia 1984, 114, 177–187. [CrossRef]
25. Arnon, D. Copper enzymes in isolated chloroplasts. Plant. Physiol. 1949, 24, 1–15. [CrossRef]
26. Kvitek, L.; Panacek, A.; Soukupova, J.; Kolar, M.; Vecerova, R.; Prucek, R.; Holecova, M.; Zboril, R. Effect of
surfactants and polymers on stability and antibacterial activity of silver nanoparticles (NPs). J. Phys. Chem.
C 2008, 112, 5825–5834. [CrossRef]
27. Zook, J.M.; Halter, M.D.; Cleveland, D.; Long, S.E. Disentangling the effects of polymer coatings on silver
nanoparticle agglomeration, dissolution, and toxicity to determine mechanisms of nanotoxicity. J. Nanopart.
Res. 2012, 14, 1165–1173. [CrossRef]
28. Arif, S.; Batool, A.; Khalid, N.; Ahmed, I.; Janjua, H.A. Comparative analysis of stability and biological
activities of violacein and starch capped silver nanoparticles. RSC Adv. 2017, 7, 4468–4478. [CrossRef]
29. Hebeish, A.A.; El-Rafie, M.H.; Abdel-Mohdy, F.A.; Abdel-Halim, E.S.; Emam, H.E. Carboxymethyl cellulose
for green synthesis and stabilization of silver nanoparticles. Carbohyd. Polym. 2010, 82, 933–941. [CrossRef]
30. Dai, Y.; Deng, T.; Jia, S.; Jin, L.; Lu, F. Preparation and characterization of fine silver powder with colloidal
emulsion aphrons. J. Membr. Sci. 2006, 281, 685–691. [CrossRef]
31. Wang, H.; Qiao, X.; Chen, J.; Ding, S. Preparation of silver nanoparticles by chemical reduction method.
Colloid. Surf. A 2005, 256, 111–115. [CrossRef]
32. Ahila, N.K.; Ramkumar, V.S.; Prakash, S.; Manikandan, B.; Ravindran, J.; Dhanalakshmi, P.K.; Kannapiran, E.
Synthesis of stable nanosilver particles (AgNPs) by the proteins of seagrass Syringodium isoetifolium and its
biomedicinal properties. Biomed. Pharmacother. 2016, 84, 60–70. [CrossRef] [PubMed]
33. Ahmad, N.; Sharma, S. Green synthesis of silver nanoparticles using extracts of Aananas comosus. Green
Sustain. Chem. 2012, 2, 141–147. [CrossRef]
34. Gondikas, A.P.; Morris, A.; Reinsch, B.C.; Marinakos, S.M.; Lowry, G.V.; Hsu-Kim, H. Cysteine-induced
modifications of zero-valent silver nanomaterials: Implications for particle surface chemistry, aggregation,
dissolution, and silver speciation. Environ. Sci. Technol. 2012, 46, 7037–7045. [CrossRef] [PubMed]
35. Kawashima, Y.; Handa, T.; Kasai, A.; Takenaka, H.; Lin, S.Y.; Ando, Y. Novel method for the
preparation of controlled-release theophylline granules coated with a polyelectrolyte complex of sodium
polyphosphate-chitosan. J. Pharm. Sci. 1985, 74, 264–268. [CrossRef]
36. Weng, S. Fourier Transform Infrared Spectrum Analysis, 2nd ed.; Chemical Industry Press: Beijing, China, 2010;
pp. 377–388.
37. Makarov, V.V.; Love, A.J.; Sinitsyna, O.V.; Makarova, S.S.; Yaminsky, I.V.; Taliansky, M.E.; Kalinina, N.O.
“Green” nanotechnologies: Synthesis of metal nanoparticles using plants. Acta Nat. 2014, 6, 35–44. [CrossRef]
38. Zhang, Z.; Zhao, B.; Hu, L. PVP protective mechanism of ultrafine silver powder synthesized by chemical
reduction processes. J. Solid State Chem. 1996, 112, 105–110. [CrossRef]
39. Carotenuto, G.; Pepe, G.P.; Nicolais, L. Preparation and characterization of nano-sized Ag/PVP composites
for optical applications. Eur. Phys. J. B 2000, 16, 11–17. [CrossRef]
40. Zhang, C.; Hu, Z.; Deng, B. Silver nanoparticles in aquatic environments: Physiochemical behavior and
antimicrobial mechanisms. Water Res. 2016, 88, 403–427. [CrossRef]
41. Wang, P.; Zhang, B.; Zhang, H.; He, Y.; Ong, C.N.; Yang, J. Metabolites change of Scenedesmus Obliquus
exerted by AgNPs. J. Environ. Sci. 2019, 76, 310–318. [CrossRef]
Nanomaterials 2020, 10, 1042 15 of 15
42. Hotze, E.M.; Phenrat, T.; Lowry, G.V. Nanoparticle aggregation: Challenges to understanding transport and
reactivity in the environment. J. Environ. Qual. 2010, 39, 1909–1924. [CrossRef] [PubMed]
43. Liu, J.; Legros, S.; von der Kammer, F.; Hofmann, T. Natural organic matter concentration and hydrochemistry
influence aggregation kinetics of functionalized engineered nanoparticles. Environ. Sci. Technol. 2013, 47,
4113–4120. [CrossRef] [PubMed]
© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).