investigation of a structure of new functional peptide ... · pdf fileinvestigation of a...

ISSN 2070�2051, Protection of Metals and Physical Chemistry of Surfaces, 2015, Vol. 51, No. 4, pp. 558–566. © Pleiades Publishing, Ltd., 2015.Original Russian Text © A.I. Loskutov, O.Ya. Uryupina, S.N. Grigor’ev, N.V. Kosheleva, V.B. Oshurko, E.V. Romash, I.N. Senchikhin, A.V. Falin, 2015, published inFizikokhimiya Poverkhnosti i Zashchita Materialov, 2015, Vol. 51, No. 4, pp. 411–419.

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INTRODUCTION

Achievements in development of nanotechnolo�gies, in particular, in creation of new nanocompositematerials and coatings, have increased awareness oftheir deleterious effect on the environment andhuman health. The currently used techniques ofchemical synthesis of inorganic nanostructures sufferfrom several substantial drawbacks: the use of toxicreagents, extreme conditions of synthesis (high tem�peratures and/or pressures), and strongly basic oracidic media.

Compared to these methods, biological systemsallow synthesis of nanostructures under physiologicalconditions at ambient temperatures using environ�mentally friendly reactants (“green chemistry”), whileat the same time enabling synthesis and assembly ofnanostructures. The size of nanoparticles (NPs)makes them ideal candidates for the design of newfunctional materials. Synthetic peptides (PTs) are dis�tinguished by diverse chemical structures, high ther�mal and chemical stability, and biocompatibility. Theyexhibit a good ability to bond with inorganic surfaces;therefore, they can be successfully used in synthesis ofvarious nanostructures and coatings. Despite therehaving been few studies of properties of solid�phasePT layers, it has been established that the mechanical

strength of PT NPs is higher than that of steel and, atthe same strength properties, the flexibility of PTfibers is like that of silk [1]. All these properties arisefrom the features of their molecular structure. Solid�phase coatings based on PTs have good antifrictionproperties, which strongly depend on the conforma�tion type [2]. Currently, the majority of investigationsof PTs and polypeptides have been conducted in thefield of medicine and biology, where their behavior ina liquid phase is of prime importance. In creatingfunctional biomaterials, PTs usually form a solid phaseor interact with it actively. The use of proteins and PTsin biosensors and electronic devices has been a hottopic in recent years. However, although the interfacebetween bioorganic and inorganic substances largelydetermines the characteristics of such devices, the reg�ularities of their bonding with solids and formation ofsolid�phase coatings from them are still little�studied.Data on the electronic properties of such interfaces aresparse. These interactions depend on two importantfactors: the chemistry and nanotopography of the sur�face. Creation of stable nanostructures that is capableof long�term keeping of their properties under envi�ronmental conditions is needed for successful practi�cal application of PT nanocomposite materials. Forthis reason, the questions of the stability of such struc�tures are also rather pressing [3].

Investigation of a Structure of New Functional Peptide Composite Materials with Gold Nanoparticles

A. I. Loskutova, O. Ya. Uryupinab, S. N. Grigor’eva, N. V. Koshelevaa,V. B. Oshurkoa, E. V. Romasha, I. N. Senchikhinb, and A. V. Falina

a Moscow State University of Technology STANKIN, Vadkovskii per. 1, Moscow, 127994 Russiab Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,

Leninskii pr. 31, Moscow, 199071 Russiae�mail: [email protected]

Received September 4, 2013

Abstract—A structure of new composite coatings formed at room temperature from dispersions of gold nano�particles (NPs) in an aqueous solution of a peptide (PT) Asp–Glu–Val–Asp–Trp–Phe–Asp on mica, glass,silicon, aluminum, and gold substrates was investigated by IR spectroscopy, microprobe and laser interfer�ence microscopy, and dynamic light scattering. The size of gold NPs in the solution is 40 nm. It has beenestablished that an individual PT crystallizes, forming various structures, the character of which significantlydepends on the substrate nature, surface conditions, and pH value of the medium. At pH ~ 9, a zwitterionicstate of the PT molecule breaks down and its isomeric form changes (from trans� to cis�configuration). As aresult, the forming peptide layer becomes disordered and the layer growth mechanism changes from lateral toa normal one. Globular and fibrillar structures form in the composite layers adjacent to the substrate, andpeptide layers with complicated morphology (dendrites and spherulites) grow above them. The shapes andsizes of gold NPs in the coatings and differences in the structure of PT composite coatings formed with goldand silver NPs have been discussed. The conclusion has been drawn that crystallization of NPs and nanocom�posite coatings can be controlled by changing the electric charge of the peptide molecule.

DOI: 10.1134/S207020511504022X

NANOSCALE AND NANOSTRUCTUREDMATERIALS AND COATINGS

PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 51 No. 4 2015

INVESTIGATION OF A STRUCTURE 559

The PT studied herein belongs to the class of shortPTs. Such a PT can be used both for creation of newfunctional materials in technics, for instance, organicmicroelectronics and devices based on them, and inmedicine and biology. The final structure and proper�ties of solid�phase coatings cannot be predicted onlyfrom the structure of the PT molecule, all the more sowhen NPs of different metals are involved. Our inves�tigations has shown that composite materials andcoatings based on PTs and silver NPs in fact exhibitvarious structural, electrophysical, and tribologicalproperties, which make them promising as functionalelements of organic microelectronics, antifrictioncoatings for MEMS/NEMS, etc. [4, 5].

The aim of the present work was to establish linksbetween changes in the molecular structure of PTsduring synthesis of composites with gold NPs andchanges in the structure and morphology of theformed solid�phase nanocomposite coatings. Theeffect that nature of the substrate and the mechanismof formation of PT coatings have on the crystallizationprocess was also of interest.

1. EXPERIMENTAL

An amyloid type oligopeptide consisting of sevenamino�acid residues, two of which contain cyclicstructures—phenolic and indole groups—was usedfor the investigations. Its chemical formula is Asp–Glu–Val–Asp–Trp–Phe–Asp, the molecular weightis 924, and the pH value of a 0.1% solution is 5.0,which corresponds to an isoelectric point of a PT. Atpresent, most studies have dealt with short dipeptides.The PT herein has been chosen due to the features ofits molecular structure (the presence of cyclic groups),which allow one to obtain various structures owing toself�assembling and self�organization of moleculescompared to dipeptides [6]. We have not found anyinformation on analogous investigations with this PTin the literature.

Dispersions of gold NPs in the medium of the stud�ied PT were obtained in situ by chemical reduction ofchloroauric acid (HAuCl3 ⋅ 3H2O). The aforemen�tioned PT was used as a reducing agent. The reductionreaction was conducted in aqueous solutions ofHAuCl3 ⋅ 3H2O with the addition of potassium carbon�ate. All reagents were of analytical grade, and HAuCl3 ⋅3H2O was special�purity grade. The solutions wereprepared in freshly prepared bidistilled deionizedwater on the day of synthesis. The solution of pure PTis designated as sample 1.

The synthesis procedure was as follows: 2.5 mL of0.4 % HAuCl3 ⋅ 3H2O solution, 10 mL of 0.1 % PTsolution, and 1.25 mL of 0.2 M potassium carbonatesolution were successively introduced into 50 mL ofbidistilled water. The reaction mixture was heated to95°C for 30–40 min with intense stirring. The pHvalue of the obtained dispersion of gold NPs in a PT(sample 3) was 8.75. The pH values were chosen to be

so high because the rate of gold reduction was veryslow at lower pH values. After completion of the reac�tion, the solution acquired a reddish pink hue.

The IR spectra were recorded on a Nicolet 6700spectrometer in a transmission mode with a resolutionof 2 cm–1 after averaging over 128 scans in a wavenum�ber range of 400–4000 cm–1. For the measurements,films were formed from the studied samples on KRS�5windows using a standard technique.

The size of the synthesized gold NPs, determinedby dynamic light scattering (DLS) on a ZetasizerNano ZS (Malvern, United Kingdom) instrument,was 40 nm. It was assumed that the gold NPs werespherical in shape, and all gold in the solution reactedwith the PT completely.

The PT solution and synthesized nanodispersionswere applied at room temperature to the surface ofpolycrystalline gold and aluminum films prepared bythermal vacuum evaporation and cleaned surfaces ofglass, single�crystal silicon, and freshly cleaved mica.The formed layers were dried in air at room tempera�ture. Solid layers were formed by two methods. In thefirst, a droplet was applied to an immovable samplesurface from a dispenser; in the second, the sameamount of dispersion was applied to a sample surfacerotating at a speed of 2000–3000 rpm. In this way, thinand ultrathin PT layers were formed. The preparedlayers differed strongly in both their thickness andstructure. In the first case, a droplet of a colloid disper�sion applied to a solid surface left a clear�cut annularspot after evaporation of the solvent.

The maximum average layer thickness determinedby optical microscopy and profilometry reached 1 μm,and the border height was several micrometers. Duringthe formation of ultrathin coatings, very thin layerswith uniform thickness with no distinct bordersresulted.

The images of the surface of composite coatingswere obtained using a universal high�vacuum atomicforce microscope (AFM), Solver HV�MFM scanningtunnel microscope (STM) (NT�MDT, Russia), andMIM�321 laser interference microscope (OOOAmfora, Russia) with a height resolution of ~0.1 nm inthe shared facilities of the State Engineering Center ofMoscow State University of Technology STANKIN.All measurements were performed at room tempera�ture in air or under vacuum of 10–5 Pa. No significantdifferences in the results were obtained in these twocases. Therefore, all subsequent measurements werecarried out at room temperature in air.

2. RESULTS AND DISCUSSION

2.1. IR Spectroscopy Investigations of Synthesis of Gold Nanoparticles

Gold NPs form in an aqueous solution through thereduction of chloroauric acid. As this takes place, PTacts as both a reducing agent and a stabilizer for the

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surface of NPs. PT molecules in the studied range ofpH and concentrations in water form associates of 60–90 nm; however, according to the DLS measurements,the number of such formations is negligibly smallcompared to the concentration of gold NPs (the countrate for PT NPs was several orders of magnitude lowerthan that for gold NPs in the colloid dispersions).Seemingly, PT does not form considerable amounts ofaggregates in an aqueous solution and exists in theform of molecular chains, exhibiting a high mobility inan aqueous medium.

Figure 1 shows IR spectra of the films of the initialPT (sample 1 (curve 1)) and dispersions of gold NPs(sample 3 (curve 3)). It is known [7] that ionization ofthe α�amino and α�carboxyl groups (in both terminaland lateral PT chains) and, accordingly, realization ofrotational isomers in various conformations of a PTchain depend on the pH value of the medium. It is alsoknown that PTs are hydrolyzed at high pH values andtemperature. Therefore, in addition, an IR spectrumof a PT layer obtained from the solution (sample 2)containing the same amount of potassium carbonate(pH 8.75) and subjected to the same thermal treat�ment as sample 3 was recorded. This spectrum isshown as curve 2.

The lines in the PT IR spectra were identified usingthe literature data [8–1]. From curve 1 in Fig. 1, it isseen that the initial PT has a dipolar zwitterionic

structure. Stretching vibrations of ionized and

groups (terminal in the indole lateral substitu�ent) yield strong and broad absorption bands with

maxima at 3081, 2740, and 2611 cm–1. For the group, a band at 3080 cm–1 is considered to be themain maximum, while a weaker band at 2114 cm–1 isalso attributed to one of vibrations of this group by dif�ferent authors [9]. The vibrations of ionized carboxyl

NH3+

NH2+

NH3+

groups (a PT molecule contains five of them) result inthe most intense bands of the PT spectrum. The bandwith a maximum at 1592 cm–1 is ascribed to asymmet�ric stretching vibrations of an ionized carboxyl group.A band at 1411 cm–1 corresponds to symmetric vibra�tions of ionized carboxyl groups. Note that strongabsorption bands of amide I (1656 cm–1) and amide II(1550 cm–1) appear only as shoulders of the mostintense spectral band at 1592 cm–1. The presence ofthe amide II band in the spectrum of the studied PTwith a maximum at 1550 cm–1 unambiguously indi�cates that the molecule has a trans�configuration. Thepresence of PT groups and ionized α�amino andα�carboxyl groups in the PT molecule creates the con�ditions for the formation of a crystalline packet. Infact, the bands of skeletal vibrations of the PT chainare distinct in the low�frequency domain, while sev�eral bands show typical splitting. This is an effect ofresonance interaction of identical vibrational levels ofneighboring molecules, which shows up only in ahighly ordered crystalline state.

The spectrum of PT subjected to thermal treatmentobtained from the solution with an elevated pH valueof 8.75 undergoes considerable changes, as one shouldexpect (Fig. 1, curve 2). A broad band of stretchingvibrations of un�ionized NH and NH2 groups boundby hydrogen bonds with a maximum at 3400 cm–1

arises; i.e., a zwitterionic state disappears. The intenseabsorption band of ionized carboxyl groups becomesbroad and complex, composed of several bands, whichis indicative of various charges of ionized carboxylgroups. In the domain of skeletal vibrations of a PTchain, broadening, rather than splitting, of bands alsotakes place and their intensity redistributes. However,there are no absorption bands due to vibrations of anun�ionized carboxyl group. Evidently, an increase inthe pH value of the medium leads to destruction of anordered PT structure and changes its conformation;however, hydrolysis is not prominent due to either anincreasing temperature or thermal treatment (notethat we studied especially the absorption by potassiumcarbonate in the IR domain: the intensity of the stron�gest absorption band in curve 2 is at the level of noise).

Curve 3 depicts the IR spectrum of sample 3 (goldNPs in PT). Comparing it with the previous spectrum(curve 2), we note a higher order of the system, withabsorption bands in the domain of skeletal vibrationsof a PT chain becoming narrower and sharper; how�ever, there is still no splitting of bands typical of a crys�talline state. Metal reduction in the studied systemoccurs most likely via amino groups of PT. In fact, theintensity of a broad band arising due to stretchingvibrations of NH and NH2 groups decreases consider�ably compared to the spectrum of thermally treatedPT (curve 2). In the domain of absorption of stretchingvibrations of carboxyl groups and bands amine I andamine II in the spectrum of the studied sample, a sig�nificant redistribution of band intensities and widths

500100015002000250030004000 3500

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40

2611 21

14

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Fig. 1. IR spectra of PT and a composite with gold NPs:(1) the initial PT (pH 5.0), (2) PT with potassium carbon�ate subjected to thermal treatment (pH 8.75), and (3)composite of PT with gold NPs.



takes place: the most intense band in the PT spectrumat 1592 cm–1 (νas СО)) in curve 2 becomes only ashoulder of a band at 1620 cm–1 in curve 3. Therefore,gold NPs formed by reduction interact with PT car�boxyl groups. The exact positions of the maxima of theabsorption bands νas and νs CO and the values of split�ting depend on both the nature of the metal boundwith these groups and the type of bond (ionic orhydrogen: monodentate, bidentate�cyclic, bidentate�bridge, etc.) and its symmetry. In [12–15], the absorp�tion bands belonging to various types of the above�mentioned compounds are summarized. In mostcases, the bands in the region of 1600 (νas СО) and1400 cm–1(νs СО) characterize a simple ionic bond. Inthe spectrum of the “gold–PT” system, absorption inthis domain is also present; accordingly, ionic com�pounds between a carboxyl group and metal have alsoappeared in the synthesized dispersion. Unfortunately,absorption bands of other structures which a metalforms with carboxyl groups in the given domain over�lap with amine I and amine II bands; therefore, theirunequivocal identification in this domain is impossi�ble. Note that all absorption bands in this domain arebroad, which is evidence of their overlapping. Interest�ingly, a new most intense band appears at 1240 cm–1 inthe spectrum of sample 3. We observed absorption inthis region earlier with gold NPs synthesized in solu�tions of various cellulose derivatives [16], in whichreduction of gold occurred with the participation ofhydroxyl groups of cellulose molecules, and the sur�face gold NPs was stabilized by specific interactionbetween a carboxyl group and a gold NP surface. Thisband has most probably arisen here due to vibrationsof new specific bonds between gold NPs with PT func�tional groups.

Therefore, it may be considered that the absorptionband at 1240 cm–1 and a complex form of other bandsin the given domain of the spectrum are evidence ofthe formation of different bonds between carboxylgroups of a PT molecule and a metal surface, i.e., withgold NPs, which results in their stabilization.

2.2. Structure and Morphology of Peptide Layers

The investigation of ultrathin layers makes it possi�ble both to obtain coatings with more uniform thick�ness and, in some cases, to determine features of theinitial stages of crystallization and details of theirmolecular structure [17]. Figure 2 shows AFM imagesof ultrathin PT layers obtained at cleaved mica andglass substrates. It is seen that, at the mica surface, PTdoes not form a uniform layer and main structural ele�ments are variously oriented flat rods (Fig. 2a). Inaddition, at some regions of the surface, globules formnear PT islands. The minimum height of islands rela�tive to uncovered surface regions, which was deter�mined from the profile height, is 4 nm. In some cases,the details of a fine structure of rods are visible (theinset in Fig. 2a). The minimal height of rods is about10–15 nm, the width is 100–150 nm, and the length is~1 μm. The surface roughness of the rods is 0.5 nm. Wesee that the rods consist of close�packed thin plates ~6nm in thickness oriented normally to the substrate sur�face. Note that the number of such rods is consider�ably lower than that of globules and they can form onlyat a mica surface.

In ultrathin PT layers on a glass substrate, extendedflat globules with different shapes, which form fiberlikestructures, are the main structural element (Fig. 2b).unlike PT layers at the mica surface, the details of afine structure of such globules are indistinguishable.With an increasing thickness of a PT layer variouslyoriented fibrillar structures grow together forming acompact layer with well�marked “intergranular”boundaries. In the case of silicon, aluminum, andgold, flat globules are the main structural elements, ason the glass surface.

Among the studied materials—mica, glass, silicon,gold, and aluminum—the thickness of PT layersformed under analogous conditions is minimum forthe mica surface.

The structure of thin PT layers at differing sub�strates formed by droplet evaporation was investigatedprincipally in [5]. However, in this work, the data were

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Fig. 2. AFM images of ultrathin PT layers at different regions of (a) mica and (b) glass surfaces. The inset size is 100 × 40 nm2.

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obtained at pH 5.0, which corresponds to an isoelec�tric point, and the influence of the pH value and tem�perature on the PT layer structure was not considered.This was especially important, since synthesis of goldNPs occurred at rather high pH values of ~9. Theanalysis of AFM images obtained with pH changingfrom 5 to 10 revealed that, with an increasingpH value, the mechanism of nucleation and growth ofPT layers changes. In the pH range of 5–7, fibrillarand globular structures form predominantly due to theinteraction between PTs with the substrate, in whichfibrils are oriented parallel to the surface. In the rangeof intermediate pH values of 7–9, both globular andfibrillar structures exist. The further increase in thepH value to 10.0 causes accelerated growth of individ�ually located PT aggregates with differing shapes alongthe surface normal (Fig. 3). As this takes place, a uni�form layer does not form, the height of the aggregatescan achieve 1.5 μm, and their lateral sizes can be ashigh as several tens of micrometers.

2.3. Peptide Layers with Gold Nanoparticles

The structure and morphology of ultrathin layersformed at a mica surface from PT composites withgold NPs change significantly: rods disappear, anddisk�shaped flat particles partially covered with thinPT layers form (Fig. 4a). In addition, islands, appar�ently of PT molecular chains, with a height of 4–5 nmemerge, between which regions of an uncovered micasurface are observed.

In composite ultrathin layers applied to a cleanedglass surface, formation of both globular and fibrillarstructures is observed (Fig. 4b). The structure of glob�ular layers is close to that displayed in Fig. 4a.

At some surface regions, self�assembling of largeglobules ~250 nm in size sets in, resulting in formationof cross�shaped dendrites above a globular layer. How�ever, they also remain relatively flat (the ratio of aheight to diameter is ≤0.1). The globule size changes from60 to 100 nm, and their average height is 10 to 15 nm. Theglobule size grows somewhat in the direction from the

center of the layer to its periphery. The surface struc�tures of the composite layers at gold and aluminum areanalogous to those at a glass surface. At the surface ofa composite layer applied to silicon, spherulitic struc�tures result (Fig. 4c). Note that, among all studiedsubstrate materials, silicon surface wettability turnedout to be lowest.

Hence, at the initial stages of crystallization of PTcomposite layers with gold NPs, a near�surface globu�lar layer forms, above which PT aggregates grow next.

Figure 5 displays the AFM images of thin compos�ite layers formed through a “droplet evaporation”technique. The main features of this method includemore equilibrium growth conditions of layer forma�tion and stronger dependence of the layer structure onthe distance from the droplet center. For instance, inthe direction from the droplet center to its border, theaverage layer thickness increases reaching a maximumat the border, along with changes in the structure. Itwas found out that fixation of a contact line of theinterface solid–liquid–gas occurs only when a PTlayer forms in the presence of gold NPs.

In analysis of the data presented in Figs. 2–5, firstof all, strong dependence of the layer structure on thesubstrate material attracts attention. The growth pro�cess that we observe in ultrathin nanocomposite layerscontinues with subsequent formation of PT dendriteswith different shapes. This process is most pronouncedat aluminum, gold, and silicon surfaces and is less evi�dent at mica and glass surfaces. In the case of silicon,spherulitic structures, which started to form during theformation of ultrathin composite layers, remain; how�ever, globules start to grow above them.

DISCUSSION OF RESULTS

The results suggest the excellent ability of PT toform ordered self�organized structures. However, awide variety of structures of PT coatings and theirstrong dependence on the medium pH value, methodof application of a coating, substrate material, andlocation of the measured surface region significantlyhinder preparation of coatings with uniform thicknessand desired properties, as well as description of struc�tures and establishment of the mechanism of their for�mation.

The initial PT at pH 5.0 yields ordered crystallinelayers [5]. With an increasing pH to 9.0, an ordered PTstructure is destroyed and a PT conformation changesfrom trans� to cis�; however, no prominent hydrolysisoccurs due to neither an increase in temperature northermal treatment at 95°C. A change in the total PTmolecule charge due to an increase in the pH mediumvalue during synthesis of gold NPs in the solution andsubsequent stabilization of their surface call forthdestruction of crystallinity of the formed solid layers.

The studied PT belongs to a class of polyzwitteri�ons, since its molecule contains functional groupsbearing both positive and negative charges. From the

10 µm

Fig. 3. 3D AFM image of a PT layer on a glass substrate atpH 10.0. The maximum height is ~0.6 µm.



spectral and microprobe measurements, it follows thatchanges in the PT molecule total negative electriccharge and the mechanism of crystallization of PT andits composites with gold NPs are directly linked. Anincrease in the medium pH value to ~9 leads todestruction of a PT molecule zwitterionic state due todeprotonation of amino groups and decreasing of thepositive molecule charge. As a result, one of the essen�tial conditions of molecule self�organization is vio�lated: electrostatic attraction of oppositely chargedfunctional groups changes to Coulomb repulsion ofsimilarly charged groups [18]. All this impedes the for�mation of well�ordered structures with a complexmorphology.

As this takes place, we see from Fig. 3 that the layergrowth mechanism passes from the lateral one to apredominant growth along the surface normal withformation of shapeless PT aggregates.

Significant dependence of the morphology of PTlayers on the nature of the substrate and conditions onits surface are evidence of an important role of inter�phase interactions in the growth process. It shows upmost vividly in the case of ultrathin PT layers on afreshly cleaved mica surface, which is most activechemically.

Earlier, we have found out that self�organization ofgold NPs in composite layers based on cellulose deriv�atives is observed in the absence of strong interactionsbetween particles and the substrate under the condi�tions of bad substrate wetting and a low rate of solventevaporation [19]. A stronger interaction of negativelycharged functional groups of a PT molecule with amica surface, which, unlike glass, aluminum, and sili�con, contains a large number of К+ ions, seems to pre�vent formation of extended ordered structures. Itshould be also considered that aluminum and siliconsurfaces oxidated.

The presence of gold NPs largely affects the forma�tion of ordered PT layers. PT NPs instantaneouslyproduce a multitude of extra crystallization centers,which yields mainly dendritic structures rather thanglobular ones. In addition, NPs can serve as a simplemechanical obstacle to oriented growth of ordered PTlayers. Binding of PT molecules with a gold NP sur�face also changes its total electric charge and caninterfere with the formation of ordered structures. As aresult, crystallinity of PT layers breaks down and thediversity of their morphology decreases compared tothe initial PT.

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Fig. 4. AFM images of an ultrathin composite PT layer at the surface of (a) mica, (b) glass, and (c) silicon. The inset sizes are (a)2 × 2 µm2 and (b) 160 × 160 nm2.

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In formation of strongly branched PT dendrites, theangle between the growth direction of the main and lat�eral branches is 90° (Fig. 5d). This is typical of the diffu�sion�limited crystallization of polymers, in which thecrystal growth rate is determined by the rate of diffusionof macromolecules from a liquid phase to the crystalgrowth front, and the probability of attachment to differ�ent regions of the front is almost equal [20].

The formation of well�ordered diverse supramolec�ular structures above globular layers (dendrites, spher�ulites, and ribbons), which increase surface freeenergy, is kinetically more favorable. However, suchstructures are metastable and, with increasing expo�sure of applied PT coatings to a humid atmosphere,they break down [3]. Diffusion of PT chains leads tostructural relaxation with the formation of a thermo�

dynamically more favorable stable globular structure.The presence of gold NPs in a PT matrix acceleratesthese processes significantly. In addition to dendriticstructures, fibrillar structures composed of individualPT “fibrils” analogous to those presented in Figs. 2aand 4b were observed. It is well known [6] that PTmicrofibrils are bunches of protofibrils 3 nm in diam�eter; however, we did not find such fibrils.

The results of DLS measurements indicate that thesize of gold NPs is 40 nm. However, the minimal par�ticle size obtained by microprobe measurements wasconsiderably greater. The particle shape is equallyimportant. It can be determined from the aspect ratio(of height to diameter), which should be ~0.5 forspherical particles. In our case, it is ≤0.1, which corre�sponds to a flat disk�shaped particle rather than a

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Fig. 5. AFM images of thin PT layers with gold NPs at the surface of (a) mica, (b) glass, (c) silicon, (d) aluminum, and (e) gold.



spherical one. The particle shape in the solution canbe determined by DLS at differing angles of light inci�dence. However, our standard equipment does nothave such capabilities. At the same time, the instru�ment software uses the Mie theory [21], whichassumes a spherical particle shape in calculations, forwhich reason we can also consider that gold particleshave a disklike shape. Earlier measurements of sizes ofsilver particles in PT layers showed satisfactory agree�ment between the particle shape in a solution and insolid layers [5]. However, in this case, the concentra�tion of silver NPs was an order of magnitude greaterthan that of gold NPs in the solution, while PT wasused only for stabilization of a silver NP surface, ratherthan for reduction of silver ions.

In the absence of a possibility of local chemicalanalysis of the surface, there exist other, indirectmethods for determining it—measurements of distri�bution of a surface potential (SP), lateral and adhesiveforces, and phase contrast [5, 23]. Comparative mea�surements of SP distribution at the surface of PT layerswith gold NPs and without them were taken.

It was established that the values of SPs at the PTlayer surface without gold particles differ inconsider�ably (≤0.05 V) and weakly depend on the surface relief.Introduction of gold NPs into a PT matrix does notchange the character of SP distribution and the shapeof tunnel current�voltage characteristics (conduc�tance mechanism) significantly and has almost noeffect on the value of lateral and adhesive forces.

These data substantially differ from those obtainedin investigation of the “PT–silver NP” system, wherea link between the SP changes, which reached 0.7–0.8 V,and the presence of NPs on the studied surface, alongwith a considerable decrease in the local band gapwidth of a composite layer, was established. Therefore,the conclusion was drawn that silver NPs are present atthe surface of a composite layer. The differences in theproperties of composite materials with silver and goldNPs, in our opinion, are closely connected with dif�ferences in the mechanisms of NP synthesis. In fact, inthe case of silver, reduction of metal occurs with theinvolvement of a tannin molecule, which, along withPT, can be a stabilizer of a NP surface. In synthesis ofgold NPs, PT acts as both a reducing agent and an NPsurface stabilizer. Such differences in the technique ofpreparation of NPs seem to bring about variations instructure and properties of composite materials. Theinfluence of difference in concentrations of silver andgold NPs in both cases also cannot be neglected.

Comparison of the results of investigations in thesetwo systems leads us to conclude that, irrespective ofthe relief, the outer surface of the composite layer isformed by PT and gold NPs are located in the interiorof its matrix.

CONCLUSIONS

1. It was established that a close link exists between,on the one hand, a change in the total electric chargeof a PT molecule and, on the other hand, the mecha�nisms of its crystallization and structure of the formedlayer. Electrostatic interactions of oppositely chargedfunctional groups of PT molecules play a significantrole in self�assembling and self�organization of PTmolecules and gold NPs. These interactions cause theformation of morphologically complex branchedstructures (globules, fibrils, dendrites, and spheru�lites).

2. The main effect on these processes is on themedium pH value and, to a lesser extent, the substratenature. An increase in the pH value destroys a zwitte�rionic state of a PT molecule, while a transition fromthe lateral to normal growth mechanism of individualaggregates occurs with formation of disordered layers.

3. Introduction of gold NPs into a PT matrixdestructs the crystallinity of PT layers and decreasesthe diversity of their morphology compared to the ini�tial PT.

4. A change in the total electric charge of amino�acid residues enables switching on and off separatesections of a polypeptide chain during molecularinteractions. Therefore, a real possibility of control�ling the crystallization and formation of various struc�tures arises.

ACKNOWLEDGMENTS

This study was supported by the Ministry of Educa�tion and Science of the Russian Federation, govern�ment order no. 1678.

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Translated by Z. Smirnova

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