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The main objective of the present paper is the green synthesis of colloidal silver by ultrasonication starting from silver nitrate and using soluble starch as the reducing agent. Soluble starch has been used during synthesis because it is a cheap and environmentally friendly reactive. Silver colloid has been characterized by physicochemical methods: UV–VIS spectroscopy, Scanning Electron Microscopy and Energy Dispersive X-Ray spectroscopy. This colloidal material was prepared in order to prove and establish its toxicity on heterotrophic bacteria. Toxicity tests were carried out using test cultures with and without silver colloid with different concentrations. This way was possible to establish the minimum silver concentration that presents a toxic effect against used bacteria. Quantitative evaluation of bacterial growth was performed by using the Most Probable Number method. By counting the bacterial colony number, the antibacterial effect was determined for colloidal silver deposited onto the cotton gauze by adsorption. During the present study, we optimized the adsorption specific parameters: solid:liquid ratio, temperature, contact time, colloidal silver concentration. By thermodynamic, equilibrium and kinetic studies, the adsorptive process mechanism was established.
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materials
Article
New Generation of Antibacterial Products Based on
Colloidal Silver
Bogdan Pascu, Adina Negrea *, Mihaela Ciopec *, Corneliu Mircea Davidescu, Petru Negrea,
Vasile Gherman and Narcis Duteanu
Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University of Timisoara, 2 Piata
Victoriei, RO 300006 Timisoara, Romania; ioan.pascu@upt.ro (B.P.); corneliu.davidescu@upt.ro (C.M.D.);
petru.negrea@upt.ro (P.N.); vasile.gherman@upt.ro (V.G.); narcis.duteanu@upt.ro (N.D.)
*Correspondence: adina.negrea@upt.ro (A.N.); mihaela.ciopec@upt.ro (M.C.)
Received: 15 February 2020; Accepted: 25 March 2020; Published: 29 March 2020


Abstract:
The main objective of the present paper is the green synthesis of colloidal silver by
ultrasonication starting from silver nitrate and using soluble starch as the reducing agent. Soluble
starch has been used during synthesis because it is a cheap and environmentally friendly reactive.
Silver colloid has been characterized by physicochemical methods: UV–VIS spectroscopy, Scanning
Electron Microscopy and Energy Dispersive X-Ray spectroscopy. This colloidal material was prepared
in order to prove and establish its toxicity on heterotrophic bacteria. Toxicity tests were carried
out using test cultures with and without silver colloid with dierent concentrations. This way
was possible to establish the minimum silver concentration that presents a toxic eect against used
bacteria. Quantitative evaluation of bacterial growth was performed by using the Most Probable
Number method. By counting the bacterial colony number, the antibacterial eect was determined
for colloidal silver deposited onto the cotton gauze by adsorption. During the present study, we
optimized the adsorption specific parameters: solid:liquid ratio, temperature, contact time, colloidal
silver concentration. By thermodynamic, equilibrium and kinetic studies, the adsorptive process
mechanism was established.
Keywords: colloidal silver; adsorption; band aid; ultrasonic; soluble starch; antibacterial eect
1. Introduction
Technological development from the last decade lead to extensive usage of colloidal silver
in dierent fields increasing the number of possible applications. Due to the mechanical and
physical-chemical properties, colloidal silver is used in several fields: electronics, electrotechniques,
biotechnology, bioenergy, textile and optical industries, pharmacy, medicine and environmental
protection [
1
,
2
]. An extensive literature review proved that colloidal silver has a large number of
applications: antibacterial agent [
3
,
4
], antifungal agent [
5
], antiviral agent [
6
], air disinfection [
7
],
water disinfection [
3
], ground water biological wastewater treatment [
8
] and surface disinfection
(antimicrobial paints, plastic catheters, gel formulation for specific usage, packing paper for food
preservation, clinical clothing) [3].
Schematically, colloidal silver applications are presented in Figure 1.
Materials 2020,13, 1578; doi:10.3390/ma13071578 www.mdpi.com/journal/materials
Materials 2020,13, 1578 2 of 22
Materials2020,13,xFORPEERREVIEW2of25
Figure1.Silvercolloidalapplications.
Aversatilematerialwithmultipleusesintextileandmedicalfieldsiscottonbecauseitisasoft
materialandhasgoodadsorbentproperties[2,9–11].Abigproblemforusageoftextilematerialsin
themedicalfieldisrepresentedbythefactthatalltextilematerialsareexposedtoseveralvirusesand
bacteria[12].Protectivelevelofgauzeisdependentonexternallayerhydrophobicity;suchlayer
beingcalledtheprotectivelayer.Iftheprotectivelayerisnotresistivetoamicroorganism’spresence,
itmayposeariskforthehealthofthepatients.Differentmicroorganismscangrowexponentiallyif
thetemperatureandhumidityareoptimal.Startingfromthispoint,itispossibletoreduceand
minimizetheinfectionswithdifferentpathogenagentsbyusingdifferenttextilematerialswith
improvedantimicrobialproperties[13–18].Experimentaldataprovedthatsomemetals,suchassilver
orgold,orsomemetallicoxides(titaniumoxide,zincoxideorcooperoxide)presentgood
antimicrobialactivity[2,16–29].
Bycorrelatingthisobservationwiththenecessityfordifferenttextilematerialswithgood
antimicrobialproperties,weaimtoprepareanewtypeofmedicalbandagebymodifyingacotton
bandagewithanantimicrobiallayerofcolloidalsilver.Inthenewlypreparedmedicalbandages,an
improvedantimicrobialpropertysilverlayerwasintroducedbyadsorption.
Colloidalsilvercanbepreparedusingphysicalmethods(evaporation,condensation)[30,31],or
chemicalmethods(microwavesynthesis,microemulsiontechniques,chemicalreduction)[32–35],
photochemicalmethods[36–38],biologicalmethodsusingsomemicroorganisms(Pseudomonas
stutzeri)[39,40]ordifferentplantextracts:Garciniamangostana[35],Cinnamomumtamala[41].
Suchcolloidalmaterialcanbeobtainedonlyifthecolloidstabilizationisrealizedrapidlyafter
silverreduction.Reductionprocesscanberealizedbyusingdifferentreducingagents:sodiumcitrate,
ascorbate[42],sodiumborohydride[43],elementalhydrogen,differentpolyols,N,Ndimethyl
formamide(DMF)[32],ascorbicacid,poly(ethyleneglycol)–blockcopolymers[44],hydrazine,
ammoniumformate[34].
Inthepresentpaper,solublestarchhasbeenusedasthereducingagentbecauseitisagreen
reactantandduetoitslowprice.Reductionprocesswasconductedusingultrasonicationinorderto
assureandmaintainpropercontactbetweenreagents,duetothepropagationofultrasonicwavesin
liquidmedia.Inthispropagationprocess,itwasobservedthatcompressioncyclesalternatewith
lowerpressurecycleswhenaliquidrareficationisobservedleadingatcavitationprocess[45].During
thecompressioncycle,hightemperatureiscreatedintheliquid.Inaddition,itwasobservedthat
Figure 1. Silver colloidal applications.
A versatile material with multiple uses in textile and medical fields is cotton because it is a soft
material and has good adsorbent properties [
2
,
9
11
]. A big problem for usage of textile materials in
the medical field is represented by the fact that all textile materials are exposed to several viruses
and bacteria [
12
]. Protective level of gauze is dependent on external layer hydrophobicity; such layer
being called the protective layer. If the protective layer is not resistive to a microorganism’s presence,
it may pose a risk for the health of the patients. Dierent microorganisms can grow exponentially if the
temperature and humidity are optimal. Starting from this point, it is possible to reduce and minimize
the infections with dierent pathogen agents by using dierent textile materials with improved
antimicrobial properties [
13
18
]. Experimental data proved that some metals, such as silver or gold,
or some metallic oxides (titanium oxide, zinc oxide or cooper oxide) present good antimicrobial
activity [2,1629].
By correlating this observation with the necessity for dierent textile materials with good
antimicrobial properties, we aim to prepare a new type of medical bandage by modifying a cotton
bandage with an antimicrobial layer of colloidal silver. In the newly prepared medical bandages,
an improved antimicrobial property silver layer was introduced by adsorption.
Colloidal silver can be prepared using physical methods (evaporation, condensation) [
30
,
31
],
or chemical methods (microwave synthesis, microemulsion techniques, chemical
reduction) [3235],
photochemical methods [
36
38
], biological methods using some microorganisms (Pseudomonas
stutzeri) [39,40] or dierent plant extracts: Garcinia mangostana [35], Cinnamomum tamala [41].
Such colloidal material can be obtained only if the colloid stabilization is realized rapidly after
silver reduction. Reduction process can be realized by using dierent reducing agents: sodium
citrate, ascorbate [
42
], sodium borohydride [
43
], elemental hydrogen, dierent polyols, N,N dimethyl
formamide (DMF) [
32
], ascorbic acid, poly (ethylene glycol)–block copolymers [
44
], hydrazine,
ammonium formate [34].
In the present paper, soluble starch has been used as the reducing agent because it is a green
reactant and due to its low price. Reduction process was conducted using ultrasonication in order
to assure and maintain proper contact between reagents, due to the propagation of ultrasonic waves
in liquid media. In this propagation process, it was observed that compression cycles alternate
with lower pressure cycles when a liquid rarefication is observed leading at cavitation process [
45
].
During the compression cycle, high temperature is created in the liquid. In addition, it was observed
that during cavitation the contact surface between solid support and liquid molecules increases.
Cavitation phenomenon occurred simultaneously in several points inside of the liquid, leading to high
temperatures and pressures which are responsible for further physical-chemical modifications [
46
,
47
].
Materials 2020,13, 1578 3 of 22
The aim of the present paper was to prepare a medical bandage with antimicrobial properties
by deposition of colloidal silver on cotton gauze. After preparation of such advanced medical
bandages, antimicrobial activity has been tested. These tests were carried out by studying the behavior
of environmental heterotroph microorganisms when they are exposed to dierent colloidal silver
concentrations. Heterotroph microorganisms were used because they have a good resistance to dierent
drugs, dierent structures and dierent metabolism.
2. Materials and Methods
2.1. Biocide Efect of Colloidal Silver Solution
Long time usage of silver particles proved that they present a good biocide eect against a large
spectrum of Gram-positive and Gram-negative bacteria. Starting from this observation, our aim was to
test the biocide eect of prepared colloidal silver. During the experiments we used colloidal solutions
with silver concentrations between 1.2 µg L1and 116 mg L1.
The material designed by us is a new material, therefore it is preferable for the toxicological
tests to be carried out using bacterial inoculations that come from natural environments with a
broader spectrum of Gram-positive and Gram-negative bacteria. Thus, an inoculum of water from
the Bega River has been used, which contains a wide spectrum of Gram-positive and Gram-negative
bacterial species. These bacteria were grown using a solid non-selective growth medium plate count
agar—Himedia (Enzymatic Digest of Casein 5.0 g/L; Yeast Extract 2.5 g/L; Dextrose (Glucose) 1.0 g/L;
Agar 15.0 g/L; Final pH: 7.0
±
0.2 at 298.15 K), preparing two dierent types of cultures: test cultures
were prepared on plates with dierent colloidal silver content, and control cultures prepared on plates
without colloidal silver content.
Bacteria cultures were prepared in two dierent ways, for control cultures (3 repeats) in Petri
dishes containing 10 mL plate count agar growth medium were introduced 1 mL of bacterial inoculum
and 1 mL distilled water. For test cultures (3 repeats) in Petri dishes containing 10 mL plate count
agar were introduced 1 mL bacterial inoculum, and 1 mL from colloidal solution. All used Petri dishes
were incubated at 303 K for 48 hours. After incubation the quantitative bacterial growth was estimated
using the Most Probable Number method (counting the bacterial colonies) by comparing it to the
bacterial control culture.
2.2. Antibacterial Activities
Studies of antibacterial properties of colloidal silver were conducted by using test medical bandages
on which surface colloidal silver had been deposited by using a thermostatic bath. The influence of
dierent parameters (solid material: liquid ratio, silver initial concentration and temperature) were
evaluated on the adsorptive capacity of medical bandage used during tests. Optimum ratio solid:liquid
was established by varying the number of gauze pieces (with dimension of 20
×
20 mm) between 5 and
70 for the same volume of colloidal silver (25 mL) with a concentration of 50 mg L
1
. All adsorption
experiments were carried out at pH 4 and 273 K. Time contact and temperature influences on adsorption
capacity were studied by varying the contact time between 1 and 6 hours and the temperature between
298 and 318 K for the optimum solid:liquid ratio =30 pieces of gauze: 25 mL colloidal silver at pH 4.
Equilibrium concentration was established by using dierent initial concentration of colloidal
silver (5, 10, 15, 25, 40, 50, 60 and 75 mg L
1
) for the same solid:liquid ratio =30 pieces of gauze: 25 mL
colloidal silver. All adsorptions were carried out at 298 K, pH =4 for 4 hours.
From all batch experiments medical bandages were extracted from solutions and in the obtained
solutions we determined silver residual concentration by atomic absorption spectroscopy (Varian
SpectrAA 280 FS). Obtained pieces of medical bandages were dried at 303 K for a minimum of 24 h
and tested for antimicrobial eect for further usage in the medical field.
Antibacterial tests on the prepared medical bandages were performed on Bega river heterotrophic
bacteria cultures (Gram positive and Gram negative bacteria). All bacterial cultures were obtained on
Materials 2020,13, 1578 4 of 22
a solid non-selective growth medium (plate count agar), making two types of cultures: test cultures
(3 repeats) using silver impregnated medical bandages, and control cultures in which were used
un-impregnated medical bandages with the same shape and size like the samples used in test cultures.
Medical bandage pieces were placed into the Petri dishes at a distance of 2 cm from dish walls, after that
was added the plate count agar medium and the entire system was inoculated with 600
µ
L of Bega
water, with content of heterotrophic bacteria. The obtained system was incubated at 303 K for 48 h.
After incubation we evaluated the growth of the bacteria for test cultures and for control cultures.
2.3. Synthesis of Colloidal Silver
2.3.1. Eect of Precursor: Reducer Ratio
Colloidal silver synthesis was carried out by using soluble starch (analytical purity, Merck) as
the reducing agent. An important factor during synthesis is represented by the ratio between silver
precursor, reducing agent and water. As a precursor for colloidal silver preparation, silver nitrate was
used, analytic grade from Merck. During synthesis, dierent quantities of reducing agent were used:
0.5, 1, 2 and 4 g, respectively, for the same quantities of silver precursor (4 g) and solvent volume has
been kept constant for all experiments (20 mL of distilled water). By using these quantities of reagents
we obtained ratios: 0.25:2; 0.5:2; 1:2; 2:2. During preparation, all samples were mixed for a minimum of
60 minutes at 353 K using an ultrasonic bath with a power of 320 W.
2.3.2. Eect of Ultrasonication Time
Through preliminary attempts were conducted experiments for sonication time up to 240 minutes,
but the obtained results were similar with the results obtained for 90 minutes. Based on that,
in the present research paper the sonication time was varied between 30 and 90 minutes. Eect of
ultrasonication time was studied by varying the mixing time from 30 to 90 minutes at 353 K for a ratio
of silver precursor:reducing agent =1:2.
2.3.3. Eect of Ultrasonication Temperature
Temperature influence has been studied by varying the temperature between 333 and 353 K, for an
ultrasonication time of 60 minutes using a ratio of silver precursor:reducing agent:water =1:2:10.
2.4. Characterization of Colloidal Silver Materials
Presence of silver particles into the prepared silver colloid was proved by recording the UV–VIS
spectra using Varian Cary 50 spectrophotometer, and by recording the scanning electron micrographs
using a Quanta FEG – 250 Scanning Electron Microscope.
2.5. Kinetics and Thermodynamic Studies
Kinetics of the colloidal silver adsorption process on medical bandages have been studied by
modeling obtained experimental data with two kinetic models: pseudo-first-order model [
48
] and
pseudo-second-order one [49,50].
It is well known that the activation energy represents the minimum kinetic energy of reactants in
order to react. Activation energy can be evaluated from Arrhenius equation:
ln k2=ln AEa
RT (1)
where: k
2
—speed constant (g min
1
mg
1
), A—Arrhenius constant (g min mg
1
), E
a
—activation
energy (kJ mol1), T—absolute temperature (K), R—ideal gas constant (8314 J mol1K1).
Materials 2020,13, 1578 5 of 22
Adsorption processes can be characterized and optimized by evaluating the free Gibbs energy,
enthalpy and entropy. Values of thermodynamic function Gibbs free energy (
G
0
) can be calculated by
using the following relation:
G=HT·S(2)
where: G0—free Gibbs energy standard variation (kJ mol1), H0—standard enthalpy variation (kJ
mol1), S0—standard entropy variation (J mol1K1), T—absolute temperature (K).
From linear dependence ln k
d
versus 1/T(depicted in Figure 11) were evaluated the variations of
standard enthalpy and entropy:
ln Kd=
S
R
H
RT (3)
where:
H
0
—standard enthalpy variation (kJ mol
1
),
S
0
—standard entropy variation (J mol
1
K
1
),
T—absolute temperature (K) and R—ideal gas constant (8314 J mol1K1).
Equilibrium constant was calculated as the ratio between the equilibrium adsorption capacity and
the equilibrium concentration:
Kd=qe
Ce(4)
Adsorption isotherms represent a useful tool for adsorption process analysis and optimization.
Material maximum adsorption capacity was evaluated by modeling obtained experimental data with
three dierent adsorption isotherms: Langmuir isotherm (monolayer adsorption), Freundlich isotherm
(developed for heterogeneous surfaces) and Sips isotherm which at limits can describe Langmuir of
Freundlich isotherms [
51
54
]. Kinetic models are used to identify the adsorption mechanism and to
determine speed-limiting stages, including mass transport processes and chemical reactions [55].
Langmuir isotherm is based on three hypotheses: adsorption process is a monolayer adsorption,
all superficial active sites are identical, housing one single metallic ion, capacity of one molecule to be
adsorbed on surface is independent of the occupancy of the neighboring places [52,56].
Nonlinear form of Langmuir isotherm is expressed by equation [52]:
qe=qLKLCe
1+KLCe(5)
where: q
e
—equilibrium adsorption capacity (mg g
1
), C
e
—metallic ion equilibrium concentration
(mg L1), qL—Langmuir maximum adsorption capacity (mg g1), KL—Langmuir constant.
Dimensionless constant R
L
is characteristic for Langmuir isotherm, being called separation factor
or equilibrium parameter, which can be evaluated by using relation:
RL=1
1+KLCo (6)
where: R
L
—separation factor, K
L
—Langmuir constant (L mg
1
), C
0
—silver initial concentration
(mg L1).
Freundlich isotherm is an empiric one and is used developed for heterogeneous surfaces [51]:
qe=KFC1/nf
e(7)
where: q
e
—equilibrium adsorption capacity (mg g
1
), C
e
—metallic ion concentration at equilibrium
(mg g
1
), K
F
and n
F
—characteristic constants, which can be associated with relative adsorption capacity
of adsorbent and adsorption intensity, respectively.
Sips model [
53
], which at its limits describes the Langmuir and Freundlich models, is described
by relation:
qe=qsKSC1/nS
e
1+KSC1/nS
e
(8)
Materials 2020,13, 1578 6 of 22
where: q
s
—maximum adsorption capacity (mg g
1
), K
s
—constant linked with material adsorption
capacity, ns—heterogeneity factor.
Starting from Sips isotherm parameters was evaluated a dimensionless parameter—separation factor:
RS=1
1+KSC1/nS
0
(9)
where: R
s
—separation factor, K
s
—constant linked with material adsorption capacity, n
s
—heterogeneity
factor, C0—metallic ions initial concentration.
Essential characteristics of Sips isotherms are determined by Rs values: if Rs >1 adsorption is
unfavorable, Rs =1 adsorption process is a linear one, 0 <Rs <1 adsorption process is a favorable one,
if Rs =0 adsorption is an irreversible one.
3. Results and Discussion
3.1. Synthesis of Colloidal Silver
Colloidal silver can be obtained by using chemical, physical, photochemical and biological
methods. Chemical synthesis consists of the reduction of silver precursors in the presence of some
stabilizing agents. Colloidal silver synthesis by reduction of silver nitrate is a two stage process: in first
one small silver particles are formed and in second one the dimension of these particles is increasing.
3.1.1. Influence of Precursor: Reducing Agent
The influence of the ratio of silver precursor:reducing agent during colloidal silver synthesis was
studied using 4 dierent samples with ratios: 0.2:2, 0.5:2, 1:2 and 2:2. After preparation, for all colloidal
silver samples the UV–VIS spectra were recorded (Figure 2) in the spectral range of 700–300 nm.
Materials2020,13,xFORPEERREVIEW7of25
Figure2.UV–VISabsorptionspectrafordifferentprecursor:reducingagent:waterratios.
DatapresentedinFigure2showthattherecordedUV–VISspectrapresentsalargebandaround
420nm,whichisspecificforthepresenceofcolloidalsilverparticles[2,57].
Thereddishbrowncolorbecamemoreintensesimultaneouslywiththeincreaseoftheratioof
silverprecursor:reducingagent,leadingatanincreaseofsilverconcentrationincolloidalsolution.
TheincreaseofsilverconcentrationisprovedbytheincreaseofthespecificbandfromUV–VIS
spectra.
Colloidalsilverobtainedfortheratioofsilverprecursor:reducingagentbeing2:2hasahigh
viscosity,observingthepresenceofnonreactedstarch.Fortheratioofsilverprecursor:reducing
agentbeing1:2,ithasanormalconsistence,withoutnonreactedstarch.Basedonthisobservation,
wecanconcludethatthebestresultsareobtainedbyusingtheratioofsilverprecursor:reducing
agentbeing1:2.Foranyfurtherexperimentsthisratioisused.
InfluenceofUltrasoundTime
Theoptimumsonicationtimeforthesynthesisofcolloidalsilverobtainedforthechosenratio
wasestablished(Figure3).
Figure 2. UV–VIS absorption spectra for dierent precursor:reducing agent: water ratios.
Data presented in Figure 2show that the recorded UV–VIS spectra presents a large band around
420 nm, which is specific for the presence of colloidal silver particles [2,57].
Materials 2020,13, 1578 7 of 22
The reddish brown color became more intense simultaneously with the increase of the ratio of
silver precursor:reducing agent, leading at an increase of silver concentration in colloidal solution.
The increase of silver concentration is proved by the increase of the specific band from UV–VIS spectra.
Colloidal silver obtained for the ratio of silver precursor:reducing agent being 2:2 has a high
viscosity, observing the presence of non-reacted starch. For the ratio of silver precursor:reducing agent
being 1:2, it has a normal consistence, without non-reacted starch. Based on this observation, we can
conclude that the best results are obtained by using the ratio of silver precursor:reducing agent being
1:2. For any further experiments this ratio is used.
3.1.2. Influence of Ultrasound Time
The optimum sonication time for the synthesis of colloidal silver obtained for the chosen ratio
was established (Figure 3).
Materials2020,13,xFORPEERREVIEW8of25
Figure3.UV–VISabsorptionspectrafordifferentultrasoundtimes.
DatapresentedinFigure3showthepresenceofacolloidalsilverspecificbandlocatedat
approximately420nmforallthreeusedtimes[2,57].Itcanalsobeobservedthatthebestresultsare
obtainedforsynthesisconductedfor60minutes.Basedonthisobservationwedecidedthatallfurther
syntheseswillbecarriedoutfor60minutes.
InfluenceofUltrasoundBathsTemperature
Anotherimportantparameterforcolloidalsilverpreparationissynthesistemperature.Inorder
todeterminetheoptimumsynthesistemperature,allsyntheseswerecarriedoutatthreedifferent
temperatures:333,343,353K.Afterpreparation,werecordedtheVISspectrainordertoprovethe
presenceofsilverparticlesintothepreparedcolloidalsolutions(Figure4).
Figure 3. UV–VIS absorption spectra for dierent ultrasound times.
Data presented in Figure 3show the presence of a colloidal silver specific band located at
approximately 420 nm for all three used times [
2
,
57
]. It can also be observed that the best results are
obtained for synthesis conducted for 60 minutes. Based on this observation we decided that all further
syntheses will be carried out for 60 minutes.
3.1.3. Influence of Ultrasound Baths Temperature
Another important parameter for colloidal silver preparation is synthesis temperature. In order
to determine the optimum synthesis temperature, all syntheses were carried out at three dierent
temperatures: 333, 343, 353 K. After preparation, we recorded the VIS spectra in order to prove the
presence of silver particles into the prepared colloidal solutions (Figure 4).
Materials 2020,13, 1578 8 of 22
Materials2020,13,xFORPEERREVIEW9of25
Figure4.UV–VISabsorptionspectrafordifferentultrasoundbathtemperatures.
FromtherecordedVISspectra,itwasobservedthattheincreaseoftemperaturehasabeneficial
effect,leadingtoanincreaseofsilverconcentrationincolloidalsilver.Bycorroboratingallthese
resultswiththerecordedSEMmicrographs,wecanconcludethattheoptimumtemperaturefor
colloidalsilverpreparationis353K,becauseatthistemperaturesilverparticleswithlowdimensions
andrelativelygooddispersionwereobtained.
3.2.CharacterizationofColloidalSilverMaterials
SEMandEDXCharacterization
ThepreparedmaterialwascharacterizedusingScanningElectronMicroscopy(SEM)andby
EnergyDispersiveXrayspectroscopy(EDX).SEMmicroscopywasusedtoanalyzesurface
morphology(Figure5a)andEDX(Figure5b)wasusedtoprovethepresenceofsilverionsinthe
synthesizedmaterial.
Figure 4. UV–VIS absorption spectra for dierent ultrasound bath temperatures.
From the recorded VIS spectra, it was observed that the increase of temperature has a beneficial
eect, leading to an increase of silver concentration in colloidal silver. By corroborating all these results
with the recorded SEM micrographs, we can conclude that the optimum temperature for colloidal silver
preparation is 353 K, because at this temperature silver particles with low dimensions and relatively
good dispersion were obtained.
3.2. Characterization of Colloidal Silver Materials
SEM and EDX Characterization
The prepared material was characterized using Scanning Electron Microscopy (SEM) and by
Energy Dispersive X-ray spectroscopy (EDX). SEM microscopy was used to analyze surface morphology
(Figure 5a) and EDX (Figure 5b) was used to prove the presence of silver ions in the synthesized material.
From the SEM image presented in Figure 5a, the presence of some white spots were observed
which were associated with silver presence. Analyzing the SEM picture from Figure 5we can observe
that the silver particles present a size distribution between 70.82 nm and 142.00 nm. Presence of silver
particles was proved by recording the EDX spectra, in which the peaks for silver and for starch specific
elements were present. Nitrogen presence is associated with incomplete reduction of silver nitrate,
or due to its excess. The presence of oxygen atoms in the EDX spectra can be explained by considering
that the silver adsorption onto the cotton surface has been realized in air, so during the process some
silver atoms get oxidized.
Materials 2020,13, 1578 9 of 22
Materials2020,13,xFORPEERREVIEW10of25
(a)SEMimage
(b)EDXspectra
Figure5.SEMimage(a)andEnergyDispersiveXray(EDX)spectra(b)forsynthetizedcolloidal
silver.
FromtheSEMimagepresentedinFigure5a,thepresenceofsomewhitespotswereobserved
whichwereassociatedwithsilverpresence.AnalyzingtheSEMpicturefromFigure5wecanobserve
thatthesilverparticlespresentasizedistributionbetween70.82nmand142.00nm.Presenceofsilver
particleswasprovedbyrecordingtheEDXspectra,inwhichthepeaksforsilverandforstarch
specificelementswerepresent.Nitrogenpresenceisassociatedwithincompletereductionofsilver
nitrate,orduetoitsexcess.ThepresenceofoxygenatomsintheEDXspectracanbeexplainedby
consideringthatthesilveradsorptionontothecottonsurfacehasbeenrealizedinair,soduringthe
processsomesilveratomsgetoxidized.
3.3.ApplicationoftheColloidalSilver
BiocideEffectofColloidalSilverSolution
Figure 5.
SEM image (
a
) and Energy Dispersive X-ray (EDX) spectra (
b
) for synthetized colloidal silver.
3.3. Application of the Colloidal Silver
3.3.1. Biocide Eect of Colloidal Silver Solution
Biocide character of synthesized colloidal silver was performed on a bacterial community obtained
from the environment, presenting a large biodiversity and higher resistance to environmental factors.
Figure 6shows the bacterial growth in the control and test cultures. In the control culture (without
colloidal silver), the average number of bacterial cells (colony forming unit) per mL was 9320 CFU.
In the test sample (with colloidal silver) it was observed that the bacteria grew up when colloidal
silver concentration was under 50 mg L
1
. Based on this observation it was considered that the
toxic concentration is 50 mg L
1
. Due to this, all further experiments were carried out at this
silver concentration.
Materials 2020,13, 1578 10 of 22
Materials2020,13,xFORPEERREVIEW11of25
Biocidecharacterofsynthesizedcolloidalsilverwasperformedonabacterialcommunity
obtainedfromtheenvironment,presentingalargebiodiversityandhigherresistanceto
environmentalfactors.Figure6showsthebacterialgrowthinthecontrolandtestcultures.Inthe
controlculture(withoutcolloidalsilver),theaveragenumberofbacterialcells(colonyformingunit)
permLwas9320CFU.Inthetestsample(withcolloidalsilver)itwasobservedthatthebacteriagrew
upwhencolloidalsilverconcentrationwasunder50mgL1.Basedonthisobservationitwas
consideredthatthetoxicconcentrationis50mgL1.Duetothis,allfurtherexperimentswerecarried
outatthissilverconcentration.
(a)
(b)
Materials2020,13,xFORPEERREVIEW12of25
Figure6.Bacterialinhibitionzoneimageandtotalcolonyunits.a)Control;b)50mgAgcolloidal/L;
c)CFUvscolloidalsilverconcentration.
3.3.2.AntibacterialActivities
Toprovetheantibacterialpropertiesofmedicalbandageswithsilver,colloidalsilverwas
depositedonmedicalbandagesbyadsorption.Fortheadsorptionprocessweestablishedspecific
parameters:ratiosolidadsorbent:colloidalsolution,contacttime,temperature,initialconcentration
ofcolloidalsilverandtheinfluenceoftheseparametersonadsorptioncapacityofmedicalbandages.
Inaddition,theadsorptionmechanismbyequilibrium,kineticandthermodynamicstudieswas
established.
3.3.3.InfluenceofSolid:LiquidRatio
Influenceofratiosolidadsorbent:colloidalsilveronadsorptioncapacitywasestablishedby
changingthenumbersofmedicalbandagesimmersedintothecolloidalsilversolution.Inthese
experimentswechangedthenumberofsamplesintroducedin25mLofcolloidalsilversolutionwith
aconcentrationof50mgL
1
.Experimentswerecarriedoutat298K,foracontacttimeof4handpH
4.InFigure7ispresentedtheinfluenceoftheratioofsolid:liquidoveradsorptioncapacity.
0
2000
4000
6000
8000
10000
Colonyformingunit(CFU)
Coloidalsilverconcentration
(c)
Figure 6.
Bacterial inhibition zone image and total colony units. (
a
)—Control; (
b
)—50 mg Ag colloidal/L;
(c)—CFU vs colloidal silver concentration.
Materials 2020,13, 1578 11 of 22
3.3.2. Antibacterial Activities
To prove the antibacterial properties of medical bandages with silver, colloidal silver was deposited
on medical bandages by adsorption. For the adsorption process we established specific parameters:
ratio solid adsorbent:colloidal solution, contact time, temperature, initial concentration of colloidal
silver and the influence of these parameters on adsorption capacity of medical bandages. In addition,
the adsorption mechanism by equilibrium, kinetic and thermodynamic studies was established.
3.3.3. Influence of Solid:Liquid Ratio
Influence of ratio solid adsorbent:colloidal silver on adsorption capacity was established by
changing the numbers of medical bandages immersed into the colloidal silver solution. In these
experiments we changed the number of samples introduced in 25 mL of colloidal silver solution with a
concentration of 50 mg L
1
. Experiments were carried out at 298 K, for a contact time of 4 h and pH 4.
In Figure 7is presented the influence of the ratio of solid:liquid over adsorption capacity.
Materials2020,13,xFORPEERREVIEW13of25
Figure7.Influenceofsolid(S):liquid(L)ratioonadsorptionprocessefficiency.
FromFigure7wecanobservethatwiththeincreaseofthesolid:liquidratio,adsorption
efficiencyhasincreased,untilaratioofsolid:liquid=0.3g:25mLwhenaplateauisreached.This
plateaucorrespondstoanadsorptioncapacityof70%.Thisratiocorrespondsto30pieces20×20
mmofmedicalbandages.Basedonthisobservationallfurtherexperimentswerecarriedoutbyusing
theoptimumsolid:liquidratio.
InfluenceofContactTimeandTemperature
InFigure8theinfluencesofcontacttimeandtemperatureonmaximumadsorptioncapacityare
presented.DatapresentedinFigure8revealthatthetemperatureandtimeincreasepresentasimilar
influenceonmaximumadsorptioncapacity.Whenthecontacttimeandtemperatureincrease,the
maximumadsorptioncapacityincreases.After4hmaximumadsorptioncapacityremainsconstant.
Temperatureinfluenceonmaximumadsorptioncapacitydoesnothaveasignificantinfluence,soit
isnotnecessarilytoworkattemperatureshigherthan298K.Basedontheseobservationswechose
theoptimumconditions:time—4handtemperatureof298K.
Figure 7. Influence of solid (S):liquid (L) ratio on adsorption process eciency.
From Figure 7we can observe that with the increase of the solid:liquid ratio, adsorption eciency
has increased, until a ratio of solid:liquid =0.3 g:25 mL when a plateau is reached. This plateau
corresponds to an adsorption capacity of 70 %. This ratio corresponds to 30 pieces 20
×
20 mm of
medical bandages. Based on this observation all further experiments were carried out by using the
optimum solid:liquid ratio.
3.3.4. Influence of Contact Time and Temperature
In Figure 8the influences of contact time and temperature on maximum adsorption capacity are
presented. Data presented in Figure 8reveal that the temperature and time increase present a similar
influence on maximum adsorption capacity. When the contact time and temperature increase, the
maximum adsorption capacity increases. After 4 h maximum adsorption capacity remains constant.
Temperature influence on maximum adsorption capacity does not have a significant influence, so it is
not necessarily to work at temperatures higher than 298 K. Based on these observations we chose the
optimum conditions: time—4 h and temperature of 298 K.
Materials 2020,13, 1578 12 of 22
Materials2020,13,xFORPEERREVIEW14of25
Figure8.Influenceofcontacttimeandtemperatureonadsorptioncapacity.
KineticsStudies
Basedontheobtainedexperimentaldataweestablishedthekineticmechanismforsilver
adsorptionprocess.Colloidalsilveradsorptionprocesskineticsonmedicalbandageswere
establishedbyusingtwokineticsequations:pseudofirstordermodel[48]andpseudosecondorder
model[49,50].
Experimentaldataweremodeledusingthelinearformsoftheseequations.Speedconstantfor
pseudofirstordermodelwasdeterminedfromlinearrepresentationofln(qeqt)versustime,andthe
speedconstantforpseudosecondorderwasdeterminedfromlinearrepresentationoft/qtversus
time.BasedonthemodeleddatapresentedinFigure9wedeterminedthekineticparameters
associatedwiththeadsorptionprocessforusedkineticmodels(Table1).Basedontheobtained
correlationfactorsweestablishedthekineticmodelwhichhadbetterdescribedthestudied
adsorptionprocess.
Figure 8. Influence of contact time and temperature on adsorption capacity.
3.3.5. Kinetics Studies
Based on the obtained experimental data we established the kinetic mechanism for silver adsorption
process. Colloidal silver adsorption process kinetics on medical bandages were established by using
two kinetics equations: pseudo-first-order model [48] and pseudo-second-order model [49,50].
Experimental data were modeled using the linear forms of these equations. Speed constant for
pseudo-first-order model was determined from linear representation of ln(q
e
– q
t
)versus time, and the
speed constant for pseudo-second-order was determined from linear representation of t/q
t
versus time.
Based on the modeled data presented in Figure 9we determined the kinetic parameters associated
with the adsorption process for used kinetic models (Table 1). Based on the obtained correlation factors
we established the kinetic model which had better described the studied adsorption process.
Table 1. Kinetic parameters for the adsorption of colloidal silver onto material/textile/cotton.
Pseudo-First Order
Temperature (K) qe,exp
(mg g1)
k1
(min1)
qe,calc
(mg g1)R2
298 2.16 0.246 1.06 0.9839
308 2.38 0.322 1.81 0.8906
318 2.55 0.430 2.03 0.9133
Pseudo-Second Order
Temperature (K) qe,exp
(µg g1)
k2
(g mg1·min1)
qe,calc
(mg g1)R2
298 2.16 13.52 2.58 0.9974
308 2.38 17.83 3.03 0.9803
318 2.55 21.13 3.15 0.9831
Materials 2020,13, 1578 13 of 22
Materials2020,13,xFORPEERREVIEW15of25
Figure9.Kineticstudies.(a)pseudofirstorder;(b)pseudosecondorder.
Table1.Kineticparametersfortheadsorptionofcolloidalsilverontomaterial/textile/cotton.
PseudoFirstOrder
Temperature(K)qe,exp
(mgg1)
k1
(min1)
qe,calc
(mgg1)R2
2982.160.2461.060.9839
3082.380.3221.810.8906
3182.550.4302.030.9133
PseudoSecondOrder
Temperature(K)qe,exp
(μgg1)
k2
(gmg1min1)
qe,calc
(mgg1)R2
2982.1613.522.580.9974
3082.3817.833.030.9803
3182.5521.133.150.9831
(a)
(b)
Figure 9. Kinetic studies. (a) pseudo-first-order; (b) pseudo-second-order.
Analyzing the kinetic parameters obtained when experimental data were modeled with
pseudo-first-order model, it was observed that the correlation coecient is lower than 1, meaning that
this kinetic model is not describing the studied adsorption process. In addition, we calculated the
maximum adsorption, which had lower values compared with the experimentally obtained ones.
Similarly, the experimental data were modeled with the pseudo-second-order equation in order
to prove that the model is describing the colloidal silver adsorption on medical bandages. Kinetic
parameters (Table 1) associated with pseudo-second-order were determined from linear dependence
of t/q
t
versus time (Figure 9). From the values of the correlation coecient close to the unity, it was
proved that the pseudo-second-order model is describing the studied adsorption process. Another
confirmation is represented by the values of calculated adsorption capacity which are very close to the
experimental ones.
For the studied adsorption process, the activation energy was evaluated from linear dependence of
lnk2 versus 1/T(Figure 10). In this case, the activation energy was evaluated by using the speed constant
obtained for the pseudo-second-order model, which is accurately describing the adsorption process.
Materials 2020,13, 1578 14 of 22
Materials2020,13,xFORPEERREVIEW16of25
Analyzingthekineticparametersobtainedwhenexperimentaldataweremodeledwithpseudo
firstordermodel,itwasobservedthatthecorrelationcoefficientislowerthan1,meaningthatthis
kineticmodelisnotdescribingthestudiedadsorptionprocess.Inaddition,wecalculatedthe
maximumadsorption,whichhadlowervaluescomparedwiththeexperimentallyobtainedones.
Similarly,theexperimentaldataweremodeledwiththepseudosecondorderequationinorder
toprovethatthemodelisdescribingthecolloidalsilveradsorptiononmedicalbandages.Kinetic
parameters(Table1)associatedwithpseudosecondorderweredeterminedfromlineardependence
oft/q
t
versustime(Figure9).Fromthevaluesofthecorrelationcoefficientclosetotheunity,itwas
provedthatthepseudosecondordermodelisdescribingthestudiedadsorptionprocess.Another
confirmationisrepresentedbythevaluesofcalculatedadsorptioncapacitywhichareverycloseto
theexperimentalones.
Forthestudiedadsorptionprocess,theactivationenergywasevaluatedfromlineardependence
oflnk2versus1/T(Figure10).Inthiscase,theactivationenergywasevaluatedbyusingthespeed
constantobtainedforthepseudosecondordermodel,whichisaccuratelydescribingtheadsorption
process.
Figure10.lnK
2
=f(1/T).
BasedonthedatafromFigure10wecalculatedtheactivationenergyforsilveradsorptionon
medicalbandages(17.72kJmol
1
).
ThermodynamicsStudiesoftheAdsorptionProcess
Thermodynamicstudieswerecarriedoutinthetemperaturerangeof298to318Kinorderto
confirmiftheadsorptionprocessisaspontaneousone.Valuesofthermodynamicfunctionsare
presentedinTable2.
Fromthelineardependencelnk
d
versus1/Tweevaluatedthevariationsofstandardenthalpy
andentropy(Figure11).
Figure 10. ln K2=f (1/T).
Based on the data from Figure 10 we calculated the activation energy for silver adsorption on
medical bandages (17.72 kJ mol1).
3.3.6. Thermodynamics Studies of the Adsorption Process
Thermodynamic studies were carried out in the temperature range of 298 to 318 K in order
to confirm if the adsorption process is a spontaneous one. Values of thermodynamic functions are
presented in Table 2.
Table 2. Thermodynamic parameters for adsorption of silver onto cotton.
H0(kJ/mole) S0(J/mole·K) G0
(kJ/mole) R2
5.7 19.24 298 K 308 K 318 K 0.9999
11.8 20.36 39.68
From the linear dependence ln k
d
versus 1/Twe evaluated the variations of standard enthalpy and
entropy (Figure 11).
Analyzing the data presented in Table 2, we can observe that the Gibbs free energy variation has
negative values, meaning that the studied adsorption process is a spontaneous one. Temperature
increase leads to a decrease of Gibbs free energy value, confirming that the studied adsorption process
is favored by temperature increase. By correlating the small increase of maximum adsorption capacity
with temperature increase and positive values of enthalpy, we can conclude that the studied adsorption
processes are endothermic. A positive value of entropy variation suggests that the studied adsorption
process presents a relatively high disorder at the solid/liquid interface, but the low value of entropy
suggests that major changes of disorder degree are not taking place at the liquid/solid interface.
Materials 2020,13, 1578 15 of 22
Materials2020,13,xFORPEERREVIEW17of25
Figure11.lnkd=f(1/T).
Table2.Thermodynamicparametersforadsorptionofsilverontocotton.
ΔH0
(kJ/mole)ΔS0(J/moleK)ΔG0
(kJ/mole)R2
5.719.24298K308K318K0.9999
11.8−20.36−39.68
AnalyzingthedatapresentedinTable2,wecanobservethattheGibbsfreeenergyvariationhas
negativevalues,meaningthatthestudiedadsorptionprocessisaspontaneousone.Temperature
increaseleadstoadecreaseofGibbsfreeenergyvalue,confirmingthatthestudiedadsorption
processisfavoredbytemperatureincrease.Bycorrelatingthesmallincreaseofmaximumadsorption
capacitywithtemperatureincreaseandpositivevaluesofenthalpy,wecanconcludethatthestudied
adsorptionprocessesareendothermic.Apositivevalueofentropyvariationsuggeststhatthestudied
adsorptionprocesspresentsarelativelyhighdisorderatthesolid/liquidinterface,butthelowvalue
ofentropysuggeststhatmajorchangesofdisorderdegreearenottakingplaceattheliquid/solid
interface.
EquilibriumStudies
Figure12depictsexperimentaldatamodeledwiththechosenkineticisotherms.Theobtained
kineticparametersarepresentedinTable3.
Figure 11. ln kd=f (1/T).
3.3.7. Equilibrium Studies
Figure 12 depicts experimental data modeled with the chosen kinetic isotherms. The obtained
kinetic parameters are presented in Table 3.
Materials2020,13,xFORPEERREVIEW18of25
Figure12.Experimentaldatamodeledwithadsorptionisotherms.
Table3.Kineticparametersobtainedfortheusedadsorptionisotherms.
LangmuirIsotherm
qm,exp(mg/g)KL(L/mg)qL(mg/g)R2
2.350.2392.300.95616
FreundlichIsotherm
KF(mg/g)1
/
nFR2
0.7250.290.93262
SipsIsotherm
KSqS(mg/g)1
/
nSR2
0.2682.740.30.95548
BasedonthedatalistedinTable3,itwasobservedthatathigherequilibriumconcentrationsthe
medicalbandage’smaximumadsorptioncapacitytendstobeaconstantvalue,whichisthe
experimentalmaximumadsorptioncapacity(qexp—2.35mgg1).
Theparameter1/nFhasasubunitvalue(1/nF—0.29),thereforetheusedadsorbentpresentsa
goodaffinityforcolloidalsilver.Consideringtheheterogeneityfactor1𝑛
0.3,whichrepresents
ahighdeviationfromunity,weconcludethatthemedicalbandageadsorbentpresentsa
heterogeneoussurface.Inaddition,fromthepresenteddataitcanobservedthatthelowestvalueof
thecorrelationcoefficientwasobtainedwhentheexperimentaldataweremodeledusingFreundlich
isotherm.Inaddition,itwasobservedthattheadsorptionprocessisbetterdescribedbyLangmuir
isotherm(R2=0.95616).WhenexperimentaldataweremodeledusingtheLangmuirisotherm,the
calculatedmaximumadsorptioncapacity(𝑞2.30 𝑚𝑔 𝑔
)isclosetotheexperimentallyobtained
one(𝑞, 2.35 𝑚𝑔 𝑔
 ),suggestingthattheLangmuirisothermisdescribingtheadsorption
process.
TheMaterialCharacterizationafterSilverColloidalAdsorption
Inordertoconfirmthepresenceofsilverparticlesadsorbedontothecottonmedicalbandages,
theywerecharacterizedbyusingSEMandEDX(micrographsandEDXspectraarepresentedin
Figure13a–d).
Figure 12. Experimental data modeled with adsorption isotherms.
Table 3. Kinetic parameters obtained for the used adsorption isotherms.
Langmuir Isotherm
qm,exp (mg/g) KL(L/mg) qL(mg/g) R2
2.35 0.239 2.30 0.95616
Freundlich Isotherm
KF(mg/g) 1/nFR2
0.725 0.29 0.93262
Sips Isotherm
KSqS(mg/g) 1/nSR2
0.268 2.74 0.3 0.95548
Based on the data listed in Table 3, it was observed that at higher equilibrium concentrations
the medical bandage’s maximum adsorption capacity tends to be a constant value, which is the
experimental maximum adsorption capacity (qexp—2.35 mg g1).
Materials 2020,13, 1578 16 of 22
The parameter 1/n
F
has a subunit value (1/n
F
—0.29), therefore the used adsorbent presents a
good anity for colloidal silver. Considering the heterogeneity factor
1
ns=
0.3, which represents a
high deviation from unity, we conclude that the medical bandage adsorbent presents a heterogeneous
surface. In addition, from the presented data it can observed that the lowest value of the correlation
coecient was obtained when the experimental data were modeled using Freundlich isotherm. In
addition, it was observed that the adsorption process is better described by Langmuir isotherm (R
2
=0.95616). When experimental data were modeled using the Langmuir isotherm, the calculated
maximum adsorption capacity (
qL=
2.30
mg g1
) is close to the experimentally obtained one
(qm,exp =2.35 mg g1), suggesting that the Langmuir isotherm is describing the adsorption process.
3.3.8. The Material Characterization after Silver Colloidal Adsorption
In order to confirm the presence of silver particles adsorbed onto the cotton medical bandages,
they were characterized by using SEM and EDX (micrographs and EDX spectra are presented in
Figure 13a–d).
Materials2020,13,xFORPEERREVIEW19of25
(a)
(b)
Figure 13. Cont.
Materials 2020,13, 1578 17 of 22
Materials2020,13,xFORPEERREVIEW20of25
(c)
(d)
Figure13.Characterizationofthecottonbeforeandaftercolloidalsilveradsorption.(a)SEMimage
beforeadsorption;(b)EDXspectrabeforeadsorption;(c)SEMimageafteradsorption;(d)EDXspectra
afteradsorption.
FromtherecordedSEMimages,thepresenceofsilveratomsafteradsorptioncanbeobserved
(Figure13c),whichwasalsoconfirmedbyEDXspectrabecauseofthesilverpeak.
3.3.9.AdsorptionProcessMechanismSchematicallythemechanismofcolloidalsilveradsorption
processispresentedinFigure14.
Figure 13.
Characterization of the cotton before and after colloidal silver adsorption. (
a
) SEM image
before adsorption; (
b
) EDX spectra before adsorption; (
c
) SEM image after adsorption; (
d
) EDX spectra
after adsorption.
From the recorded SEM images, the presence of silver atoms after adsorption can be observed
(Figure 13c), which was also confirmed by EDX spectra because of the silver peak.
Materials 2020,13, 1578 18 of 22
3.3.9. Adsorption Process Mechanism—Schematically the Mechanism of Colloidal Silver Adsorption
Process is Presented in Figure 14
Materials2020,13,xFORPEERREVIEW21of25
Figure14.Mechanismofcolloidalsilveradsorptionprocess.
Afterweestablishedtheoptimumconditionsforsilveradsorptiononmedicalbandages,we
testedtheantibacterialeffectinordertoproveitsusageinthemedicalfield.Inthiscasematerial
piecesweredriedat303Kfor24handusedinantibacterialtests(Figure15).
 
(a)(b)
Figure15.Antimicrobialtest.(a)Control(b)Test.
Forantibacterialtests,piecesoftheproducedmaterialwereplacedinPetridishesat2cmfrom
theedges,andwereinoculatedusing600μLofinoculumthatcontainsheterotrophsbacteriafrom
theBegariver.Afterthat,ineachPetridishwasaddedsterileplatecountagargrowthmedium.
Figure 14. Mechanism of colloidal silver adsorption process.
After we established the optimum conditions for silver adsorption on medical bandages, we
tested the antibacterial eect in order to prove its usage in the medical field. In this case material pieces
were dried at 303 K for 24 h and used in antibacterial tests (Figure 15).
Materials2020,13,xFORPEERREVIEW21of25
Figure14.Mechanismofcolloidalsilveradsorptionprocess.
Afterweestablishedtheoptimumconditionsforsilveradsorptiononmedicalbandages,we
testedtheantibacterialeffectinordertoproveitsusageinthemedicalfield.Inthiscasematerial
piecesweredriedat303Kfor24handusedinantibacterialtests(Figure15).
 
(a)(b)
Figure15.Antimicrobialtest.(a)Control(b)Test.
Forantibacterialtests,piecesoftheproducedmaterialwereplacedinPetridishesat2cmfrom
theedges,andwereinoculatedusing600μLofinoculumthatcontainsheterotrophsbacteriafrom
theBegariver.Afterthat,ineachPetridishwasaddedsterileplatecountagargrowthmedium.
Figure 15. Antimicrobial test. (a) Control (b) Test.
Materials 2020,13, 1578 19 of 22
For antibacterial tests, pieces of the produced material were placed in Petri dishes at 2 cm from
the edges, and were inoculated using 600
µ
L of inoculum that contains heterotrophs bacteria from the
Bega river. After that, in each Petri dish was added sterile plate count agar growth medium. Prepared
Petri dishes were incubated at 303 K for 48 hours. After incubation we observed a good bacterial
growth for non-impregnated medical bandages (Figure 15 a). Bacterial growth was inhibited when the
silver concentration was higher than the minimum toxic concentration.
4. Conclusions
In the present paper was proposed a new route for colloidal silver synthesis by ultrasonication,
using soluble starch as reducing agent and stabilizer. In this case it was not needed to use a pH
regulator. Synthesized colloidal silver was characterized by UV–VIS, SEM and EDX. We investigated
adsorption mechanism, and the possible applications of deposited silver particles on medical bandages.
The adsorption mechanism was investigated by kinetic, thermodynamic and equilibrium studies. Based
on obtained experimental data we determined the optimum adsorption conditions: pH—4, time =4 h,
temperature 298 K. Silver adsorption process is described by Langmuir, when the maximum adsorption
capacity was 2.30 mg g
1
it was very close to the experimental one. From thermodynamic studies, it
was proven that the silver adsorption on medical bandages is an endothermic and spontaneous process.
As a possible application, we evaluated the biocide eect of colloidal silver and medical bandages
with silver content using heterotrophs bacteria. When the silver concentration was higher than 50 mg
L
1
, it became toxic for heterotrophic bacteria. Similar for medical bandages with silver content it was
proven that the toxic concentration for heterotrophs bacteria was 50 mg of silver.
Author Contributions:
Conceptualization: A.N., P.N. and N.D., methodology: A.N., M.C. and C.M.D.,
investigations: B.P., M.C. and V.G., manuscript preparation: A.N. and N.D., supervision P.N. and C.M.D.
All authors have read and agreed to the published version of the manuscript.
Funding:
This work was supported by a grant of the Romanian Ministery of Research and Innovation, project
number 10PFE/16.10.2018, PERFORM-TECH-UPT—The increasing of the institutional performance of the
Polytechnic University of Timi
s
,
oara by strengthening the research, development and technological transfer
capacity in the field of “Energy, Environment and Climate Change”, within Program 1—Development of
the national system of Research and Development, Subprogram 1.2—Institutional Performance—Institutional
Development Projects—Excellence Funding Projects in RDI, PNCDI III.
Conflicts of Interest: The authors declare no conflict of interest.
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... Recent studies have shown excellent antimicrobial activity of AgNPs against bacteria in synergism with antibiotics at concentrations that were not cytotoxic to human cells [12]. Many scientific communities started using AgNPs in therapeutic applications especially in many commercial items like soap, catheters, and bandages to curb infection against pathogenic agents [13][14][15][16][17]. Also, some surgical instruments, face masks, and bone cementing materials are incorporated with AgNPs for their excellent antibacterial properties [13,14]. ...
... Recent studies have shown excellent antimicrobial activity of AgNPs against bacteria in synergism with antibiotics at concentrations that were not cytotoxic to human cells [12]. Many scientific communities started using AgNPs in therapeutic applications especially in many commercial items like soap, catheters, and bandages to curb infection against pathogenic agents [13][14][15][16][17]. Also, some surgical instruments, face masks, and bone cementing materials are incorporated with AgNPs for their excellent antibacterial properties [13,14]. Nowadays, silver sulfadiazine is being replaced by silver NPs for wound healing treatment [16]. ...
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