Gold Nanoparticles Stabilized by Isonicotinic Acid - Springer Link

ISSN 00360236, Russian Journal of Inorganic Chemistry, 2015, Vol. 60, No. 3, pp. 362–371. © Pleiades Publishing, Ltd., 2015. Original Russian Text © V.V. Tatarchuk, I.A. Druzhinina, A.P. Sergievskaya, V.I. Zaikovskii, L.A. Sheludyakova, P.E. Plyusnin, P.S. Popovetskii, 2015, published in Zhurnal Neorganicheskoi Khimii, 2015, Vol. 60, No. 3, pp. 412–422.

PHYSICAL CHEMISTRY OF SOLUTIONS

Gold Nanoparticles Stabilized by Isonicotinic Acid: Synthesis in Water, Dimethylformamide, Dimethyl Sulfoxide and Characterization V. V. Tatarchuka, I. A. Druzhininaa, A. P. Sergievskayaa, V. I. Zaikovskiib, c, L. A. Sheludyakovaa, P. E. Plyusnina, c, and P. S. Popovetskiia a

Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, pr. Akademika Lavrent’eva 3, Novosibirsk, 630090 Russia b Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, pr. Lavrent’eva 5, Novosibirsk, 630090 Russia c Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090 Russia email: [email protected] Received June 30, 2014

Abstract—Hydrophilic gold nanoparticles stabilized with isonicotinic acid (INA) were prepared by reduc tion of HAuCl4 with sodium borohydride in the presence of INA in water, dimethylformamide, and dimethyl sulfoxide. The main condition for the preparation of good quality particles is to maintain reagent concentra tion ratio HAuCl4 : INA : NaBH4 = 1 : 2 : (5–10) at cAu ≤ 0.5 mM. Reaction products in water are mainly spherical primary particles with gold nuclei of 2–7 nm in diameter, those in dimethylformamide are second ary coalescent particles with nuclei of spheroid or slightly faceted shape 2–12 nm in diameter and larger elon gated agglomerates from 15 to 100 nm long. Framework structures composed of chaotically agglomerated particles were obtained at HAuCl4 : INA ratio larger than 1 : 2. By the example of precipitate of particles pre pared in dimethylformamide and on the basis of data of chemical and thermal analysis, XPD, and IR spec troscopy, it was shown that the product contains gold, INA anion, acetone, water, sodium cations, and oxy gen boron compounds. The particles obtained as precipitate or concentrate are well redispersible in water and polar solvents. The particles in colloid dispersions are in aggregated state and suffer gradual coagulation and sedimentation, which are not irreversible, and dispersed state can be restored by ultrasound treatment. The particles stabilized by INA may be of interest as initial product for preparation of nanomaterials for biomed ical purposes. DOI: 10.1134/S0036023615030201

Gold nanoparticles play important role in design ing novel materials for electronic and optoelectronic devices, chemical sensors, catalysts, and for biomedi cal technologies of cell visualization, targeted drug delivery and photothermal therapy [1–11]. In certain applications, for biomedical purposes including, hydrophilic nanoparticles are required. A large num ber of protective ligands that provide spatial or/and charge stabilization of particles depending on medium polarity in solution synthesis is known [12]. Watersol uble organic mono and polyfunctional compounds containing carboxylic and amino groups—carboxylic acids and their salts, first of all citrates, amines, and amino acids—are efficient as ligands to impart hydro philicity to nanoparticles and stabilize their colloid aqueous solutions [13–16]. Heterocyclic amines are least studied among amines of different structure used as protective ligands. One of such compounds is a pyri dine derivative containing carboxyl group, 4pyridine carboxylic or isonicotinic acid (INA). This compound is known as a reagent in the synthesis of antitubercu

lous agents of INA hydrazide series, antidepressants, and certain other therapeutics. Therefore gold nano particles with protective INA coatings are of interest as potential platform for preparing materials of biomedi cal application. Isonicotinic acid was not applied previously as pro tective ligand, therefore the aim of this work is to verify this possibility in the synthesis of hydrophilic gold nanoparticles, to optimize synthesis conditions, and to compare characteristics of particles prepared in aqueous solution and nonaqueous polar media based on dimethylformamide and dimethyl sulfoxide. EXPERIMENTAL Chemicals used in the work were gold metal (+99.95%), concentrated aqueous solutions of nitric and hydrochloric acids of reagent grade, solid NaBH4 (pure grade), rectified ethanol, acetone (high purity grade), N,Ndimethylformamide, dimethyl sulfoxide, and chloroform of pure grade, isonicotinic acid (99%,

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Aldrich). Crystalline HAuCl4 ⋅ 3H2O was prepared from gold metal by common procedure [17]. Standard solutions of isonicotinic acid and HAuCl4 in water, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) were prepared from precise weight samples of the chemicals. Nanoparticles were obtained at ambient tempera ture by mixing the portions of standard solutions of NaBH4, INA, and HAuCl4 in appropriate solvent (DMF, DMSO, or water). The formation of gold nano particles of good quality was judged from correct spec trum in situ in solution on synthesis: the spectrum with one band of surface plasmon resonance (SPR) with dis tinct symmetrical maximum at λmax ~ 520–530 nm and extinction coefficient εmax about 103 L/(mol cm), as well as weak absorption in the red region. Nanoparticles prepared in DMF were precipitated from solution by centrifugation, washed with the same solvent by decantation, and stored under DMF layer. To prepare dispersions (colloidal solutions) of nano particles in other solvents, the precipitate of particles was separated from DMF by centrifugation, washed with the same solvent by decantation, and redispersed in a fresh portion of the solvent in ultrasound bath for 20 min. For chemical analysis, the precipitate of nanoparticles separated from DMF was washed twice with acetone and dried in air at ambient temperature. We failed to separate particles obtained in water from solution by centrifugation, therefore nanoparti cles in solid state were prepared from a concentrate based on a dry residue after evaporation of reaction solution in air at ambient temperature. Along with gold and INA, the concentrate contained also nonvol atile sodium salts, chloride and borates, as reaction products resulting from initial reagents: HAuCl4, NaBH4, and H2O. The concentrate was well redispers ible in water. Absorption spectra of nanoparticle solutions in UV–visible region were recorded on a SHIMADZU UV1700 spectrophotometer relative to the solvents. It was established preliminary that INA absorption becomes considerable in the wavelength region λ < 350 nm when it is present in excess in solution, while the optical density of solutions at λ > 400 nm is caused only by gold nanoparticles. The IR spectra of nanoparticle precipitate and INA in the region of 4000–400 cm–1 were registered on a SCIMITAR FTS 2000 Fouriertransform spectrome ter as KBr pellets. The hydrodynamic diameters of gold nanoparticle aggregates (dha) in colloid solutions prepared by redis persing precipitate or concentrate of particles in sol vents were measured by photoncorrelation spectros copy (PCS) at angle 90° in a 1 × 1cm quartz cuvette at ambient temperature on a Brookhaven Inst 90Plus spectrometer. Dust was removed preliminary from the solutions by passing many times through a Teflon filter with pore diameter of 0.45 µm. Standard deviation for RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

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the obtained values of hydrodynamic diameters at more than 20 measurements was not higher 0.2 nm. The morphology and diameter of gold nuclei (dAu) of particles were determined by transmission electron microscopy (TEM) on a JEOL JEM2010 instrument with accelerating voltage of 200 kV. To prepare a sam ple, a drop of treated by ultrasound colloidal solution of particles was dried on a thin porous carbon film fixed on a copper network support. The local elemen tal analysis of the particles was performed by energy dispersive Xray spectroscopy (EDXRS) on a EDAX (EDAX Co) spectrometer with Si–Li detector. Xray powder diffraction (XRD) of precipitate and nanoparticle concentrate was performed by proce dures reported in the literature [18]. Charge, electrokinetic potential, and electro phoretic mobility of gold nanoparticles were deter mined by nonaqueous electrophoresis in chloroform medium, the procedure of measurement and data treatment was published previously [19]. Chemical CHN analysis of nanoparticle products was performed on a EURO EA 3000 analyzer. Analysis for Au and Na after dissolution of a weighed sample of nanoparticle precipitate or concentrate in aqua regia, evaporation, and transmission into 2 M HCl solution was carried out by atom absorption in air–acetylene flame on a Thermo Scientific ICE 3000 Series instru ment. Synchronous thermal analysis, which simulta neously incorporated thermogravimetric measure ments, differential scanning calorimetry, and mass spectral analysis of gaseous degradation products, was accomplished on a NETZSCH Jupiter STA 449 F1 instrument equipped with Aëolos QMS 403D quadru pole mass spectrometer. Experiments were conducted under helium atmosphere at gas flow rate of 30 mL/min and heating rate 10 K/min using crucibles made of Al2O3. Experimental data were treated using Proteus Analysis standard software package [20]. RESULTS AND DISCUSSION Synthesis of nanoparticles. The results of nanopar ticle synthesis by reduction of HAuCl4 with sodium borohydride in the presence of INA were affected by reagent concentrations and their ratio in reaction mix ture. Optimization of the synthesis in DMF medium for INA concentration at constant values cAu = 0.52 mM and NaBH4/Au = 8 showed that the best quality parti cles form at concentration ratio INA/Au = 2. At this ratio, the spectrum of reaction mixture after process completion showed SPR band at λmax = 531 nm of maximal intensity, whereas in the absorption region of particle aggregates at λ > 600 nm, optical density was minimal in the range of INA/Au variation. Moreover, the colloidal solution of nanoparticle precipitate iso lated by centrifugation, washed with solvent by decan Vol. 60

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(а) A 2

2 4 6

1 3 5 7

(b)

1.0

1 2 1

0 250

0.5

500

750 (c)

A/Amax

0 250

1000 λ, nm A

500

750

1000 λ, nm

(d)

1.5 1.0 1 2 3

1.0

1 2 3 4

0.5 0.5

0 250

500

750

1000 λ, nm

0 250

500

750 λ, nm

Fig. 1. Spectra of gold nanoparticles prepared in DMF (a, b, c) and DMSO (d). (a) Solutions in DMF in situ on synthesis, l = 1 cm, cAu = 0.52 mM, NaBH4/Au = 8, INA/Au = 1 (1), 2 (2), 3 (3), 4 (4), 5 (5), 10 (6), and 50 (7); (b) solution in DMF in situ on synthesis, cAu = 0.52 mM, Au : INA : NaBH4 = 1 : 2 : 8 (1), and redispersion of the same particles in DMF (2); (c) redispersion in DMF of particles synthesized at Au : INA : NaBH4 = 1 : 2 : 8, V = 4 (1, 3) and 8 mL (2), cAu = 0.52 (1, 2) and 1.0 mM (3); (d) solutions in DMSO in situ on synthesis, l = 1 cm, NaBH4/Au = 10, cAu = 0.33 (1–3) and 3.3 mM (4), V = 3 (1, 2, 4) and 9 mL (3), INA/Au = 10 (1, 3, 4) and 100 (2).

tation, and redispersed in DMF had the same spec trum as the particles in solution on synthesis (Fig. 1). At INA/Au ratio ≥ 3, a progressed particle aggregation was observed that appeared in spectra as the growth of absorption at λ > 600 nm. This may be caused by the fact that INA molecules include two functional frag ments: heterocyclic nitrogen and carboxyl group, therefore they can behave as linkers between the parti cles thus facilitating their aggregation. Similar study of effect of reducing agent concentra tion revealed that particles with good spectral charac

teristics form at concentration ratio NaBH4/Au = 5– 10 (Table 1). Scaling up of the synthesis at optimal ratio Au : INA : NaBH4 = 1 : 2 : 8 showed that increase in reagent amount on account of double growth of vol umes of their solutions in reaction mixture does not decrease the quality of obtained particles judging from the spectra of colloidal solutions obtained by redispersing of isolated and washed particles in DMF (Fig. 1). How ever, increase by 2 times of reagent concentrations at retention of the same absolute amounts and Au : INA :

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NaBH4 = 1 : 2 : 8 ratio caused particle aggregation, which appeared in the spectrum of dispersion as absorption growth at λ > 600 nm. In DMSO medium, we observed similar effects of reagents on the results of nanoparticle synthesis. Borohydride reduction of HAuCl4 at cAu = 0.33 mM and ratio NaBH4 : INA : NaBH4 = 1 : 10 : 10 led to preparation of particles showing a spectrum with one SPR band at λmax = 511 nm (Fig. 1). The growth of INA concentration by 10 times had negative effect on the synthesis: spectrum shape changed, absorption intensity decreased in the region 500 nm and increased at λ > 550 nm, which indicates the presence of coagu lation interactions between particles. Increase in the absolute amounts of reagents by using triple portions of their solutions in mixture at the same values of cAu = 0.33 mM and NaBH4 : INA : NaBH4 = 1 : 10 : 10 led to slight red shift of λmax to 525 nm and the growth of intensity of all spectrum. On the contrary, the tenfold growth of concentrations of all the reagents at the same ratio HAuCl4 : INA : NaBH4 ≈ 1 : 10 : 10 led to considerable aggregation of particles in solution, which is evidenced by the disappearance of band of individual Au nanoparticles at λmax ~ 520 nm and appearance of wide band with λmax ~ 600 nm typical for aggregates. The synthesis in water at optimal concentration ratio of initial reagents Au : INA : NaBH4 = 1 : 2 : (5– 10) led to preparation of nanoparticle solutions with good spectral characteristics: λmax = 518 nm, εmax = 6.0 × 103 М–1 cm–1, which were stable for a long time, at least 20 days (Fig. 2). Particle characterization. TEM data confirm pre liminary assessment of nanoparticle quality on the basis of spectra of their solutions in situ on synthesis. Aggregation and agglomeration of particles to form 3D framework chaotic structure took place when opti mal reagent ratio INA/Au = 2 was exceeded (Fig. 3). Particles obtained at optimal ratio are a mixture of nanoparticles with gold nuclei of spherical or slightly faceted shape of 2 to 12 nm in diameter and as extended agglomerates from 15 to 100 nm long. At high resolution, a twin structure of particle nuclei with size about 10 nm and larger was observed, which can indicate that these particles are secondary species resulting from aggregation of smaller primary parti cles. Since the spectra of solutions at INA/Au =2 showed no coagulation interactions of particles, one can suppose that the formation of large agglomerates proceeds at the stage of nanoparticle isolation in solid state. Particles prepared in aqueous solution at ratio NaBH4 : INA : NaBH4 = 1 : 2 : 7 had spherical smaller gold nuclei of 2–7 nm in diameter; size distribution was symmetrical, average nucleus diameter and its standard deviation was dAu = 4.1 ± 1.0 nm (number of measurements N = 137). According to highresolu RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

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Table 1. Effect of reducing agent on the position of SPR band and optical density of solutions in the absorption region of individual (530 nm) and aggregated particles (600 nm) upon synthesis in DMF (cAu = 0.52 mM, cINA = 2.7 mM, T = 298 K, l = 1 cm) NaBH4/Au

λmax, nm

A (530 nm), arb. units

A (600 nm), arb. units

1 5 6 7 8 10 20

638 532 532 529 532 536 545

1.28 1.55 1.58 1.52 1.59 1.55 1.53

1.59 1.07 1.11 1.08 1.12 1.10 1.30

tion TEM (data not given), the nuclei were monocrys talline gold with facecentered cubic lattice. Xray powder diffraction showed the presence of gold crystal phase in the products of nanoparticle syn thesis in DMF and aqueous solution (Fig. 4). The sec ond product also contained crystal phase of NaCl, which is natural because it was a nanoparticle concen trate obtained by complete evaporation of water from solution after synthesis. The EDRS local analysis of particles obtained in DMF showed that spectra contained along with sup port elements (C, Cu) also strong bands of Au as well Amax 1.6

A/Amax

1.5 1.0 1.4

0

10 t, days

20

1 2

0.5

0 250

500

750

1000 λ, nm

Fig. 2. Spectra of aqueous solution of gold nanoparticles in situ on synthesis (1) and redispersed concentrate of the nanoparticles in water (2). Inset shows stability of optical density of solution at λmax = 518 nm in time (l = 0.5 cm, cAu = 0.5 mM, Au : INA : NaBH4 = 1 : 2 : 7). Vol. 60

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50 nm

10 nm

(b) Percent fraction, %

(а)

30 25 20 15 10 5 0

100 nm

(c)

1.6

4.0 dAu, nm

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50 nm

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Fig. 3. TEM image of gold nanoparticles synthesized in DMF (a–c) and water (d) at cAu = 0.5 mM, Au : INA : NaBH4 = 1 : 2 : 8 (a, b), 1 : 5 : 8 (c), and 1 : 2 : 7 (d). Inset shows particle size distributions.

4000

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(b)

Au 111 NaCl

2000 Au 200 Au 220

0 10

40 2θ, deg

70

Intensity

Intensity

1000 Au NaCl 500

0 10

Au

Au

50

30

70

2θ, deg

Fig. 4. Diffractograms of gold nanoparticle precipitate isolated on synthesis in DMF (a), nanoparticle concentrate obtained by synthesis in water (b).

as N, O, and Na. According to chemical analysis of precipitate of these particles, the content of elements is as follows (%): Au, 64.7; C, 2.50; H, 0.45; N, 0.30; Na, 2.9. Ratios H/C ≈ 2.14 and N/C ≈ 0.10, calcu

lated from elemental analysis, differ from that for INA, which has H/C = 0.83 and N/C = 0.17. This indicates in the first place that, along with isonicotinic acid, the product should include at least one organic


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Since no crystal phases except for gold metal were found by XRD in the product of synthesis in dimeth ylformamide, for example H3BO3 or Na2B4O7, one can suppose that sodium, boron, and oxygen com pounds were in amorphous state, which may be repre sented conditionally as a mixture of Na2O, B2O3, and H2O. Let us note that the propensity of acetone and oxygen boron compounds to incorporate into prod ucts isolated on the synthesis of gold nanoparticles was already noted by us previously [21, 22]. Thus, the con ditional composition of the product per 1 mole of gold was Au(C6H5O2N)x(C3H6O)y(H2O)w(Na2O)p(B2O3)q, its efficient weight calculated from analysis for gold was Meff = 304.5 g/mol Au. Calculated assessments for factors based on elemental analysis data were: x ≈ 0.065, y ≈ 0.081, w ≈ 0.291, p ≈ 0.191. Parameter q ≈ 1.118 was assessed by residual approach as a difference between Meff and the sum of weights of all components except for B2O3 divided by molecular weight of B2O3, therefore the calculated values of percentage of the elements and efficient weight for the presented prod uct composition do not differ from the values found by analysis. The elemental analysis of nanoparticle concentrate isolated after synthesis in water afforded (%): N, 1.9; C, 10.1; H, 1.6; it corresponds to ratio N : C : H ≈ 1 : 6 : 12. A conditional mixture of substances, which contains all introduced on synthesis amounts of gold, chlorine, INA, sodium, boron, and water, had approx imately the same content of the corresponding ele ments (1.7, 9, and 1.5%): Au (0.016 mmol), C6H5O2N (0.027), NaCl (0.065), Na2O (0.058), B2O3 (0.090), H2O (0.097). The results of IR spectral study of nanoparticle pre cipitate obtained in DMF as a whole agree well with the proposed substance composition. Figure 5 shows the spectra of precipitate and initial INA, Table 2 dis plays the maxima of the main bands for the precipitate RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

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2

Transmission

compound containing carbon and no nitrogen to decrease N/C from 0.17 to 0.10. In the second place, this compound should have H/C > 2 or the product should include also water and/or hydrogencontain ing inorganic compounds to increase H/C from 0.83 to 2.14. Dimethylformamide (C6H5O2N) used as a solvent does not suitable as this compound because it contains nitrogen, while acetone (C3H7ON) used for washing in combination with INA could not provide H/C = 2.14, therefore we supposed that the product contained acetone and water. Initial gold compound HAuCl4 ⋅ 3H2O, DMF, and acetone not specially dried may be the main water sources. Considerable sodium content indicates the presence of its salts, which may be present in the product as a mixture with nanoparti cle or directly involved into nanoparticle composition. Boron and oxygen compounds may be such salts, no analysis was performed for them. These compounds obviously result from the transformation of initial NaBH4.

367

4000

3200

2400 1600 ν, cm–1

800

Fig. 5. IR spectra of gold nanoparticle precipitate prepared in DMF (1) and isonicotinic acid (2).

and the known data for sodium isonicotinate hydrate (NC5H5COONa ⋅ xH2O), a number of boron com pounds (H3BO3, HBO2, NaBO2 ⋅ 2H2O), and acetone [23]. Furthermore, Table 2 exhibits wave numbers of bands in the spectrum of glassy product obtained by hydrolysis of NaBH4 in boiling water, drying first in air and next over P2O5 at ambient temperature. The com parison of these data allows us to suppose that the IR spectrum of nanoparticle precipitate shows mainly bands of mixed vibrations of INA and boron com pounds. The IR spectrum of precipitate displays very weak band at 1720 cm–1 related to stretching vibra tions ν(C=O). This indicates that the product incor – porates anion NC5H5C O 2 rather than acid itself, the spectrum of the anion shows intense bands νas and νs assigned to vibrations of COO– group at 1588, 1546 and 1413, 1398 cm–1, respectively [24, p. 218]. These vibrations provide the main contribution to absorption in the range 1560–1400 cm–1 in the spectrum of nano particle precipitate. The presence of inflexion points in the main bands of nanoparticle precipitate seems to result also from the contribution of vibrations of boron oxygen compounds and their overlapping with bands of acetone and hydroxy groups. It should be noted that the presence of INA as undisso ciated acid NC5H4COOH or anion as a salt NC5H4COONa is insignificant for the overall composition of nanoparticle precipitate because the suggested above variant Au(C6H5O2N)x(C3H6O)y(H2O)w(Na2O)p(B2O3)q may be represented as Au(C6H4O2NNa)x(C3H6O)y (H2O)w + x/2(Na2O)p – x/2 (B2O3)q. Because of the high content of nonvolatile compo nents (Au, B2O3, Na2O) in particle precipitate pre pared in DMF, overall ~94% according to assessment, Vol. 60

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Table 2. The main vibration frequencies (cm–1) for nanoparticle precipitate and product of NaBH4 hydrolysis (HP), as well as known data for acetone, NC5H5COONa ⋅ xH2O, and boron compounds [23] Nanoparticle NC5H5COONa ⋅ xH2Oa NaBO2 ⋅ 2H2Ob precipitatea 3392

3571 3404 3169

3376 3356

HBO2b

H3BO3a

(CH3)2COc

HPa

3209

3226

3414

3442 3379 3265

2963–2870d, e 1730d

–

ν(CH)

1715

ν(CO) 1610

1588 1546 1492 sh 1450

1455

νs(COO) 1421 1363

1387

1194

1105

1061 1009 863 854

δ(H2O) νas(COO) R of ring ν(B–O)

1480

1413 1398

1316 1232 1220

ν(OH)

3005 2966 2926 1657

1652–1560e

Assignment

1198

1348 1277 1132 1079 1004 866 827

884

885

1359

δ(CH)

1189

δ(BOH)

1223

1293

1093

1093

903

937

a In KBr pellet, b in Nujol, c in thin film, d weak, e no distinct maximum; sh, shoulder.

and owing to the low weight of sample (~5 mg) used in synchronous thermal analysis, mass spectrometry data on gas phase composition are not quantitative but agree qualitatively with proposed precipitate composi tion. On heating in a helium flow up to 1000°C, the weight of solid residue was ~97% of initial weight. Mass spectrometry detected no acetone and INA mol ecules in gas phase, only weak signals of H2O and N2 were detected at ~400°C, CO2 at ~500°C. Organic components seems to degrade on heating above 300– 400°C. For example, INA in a sealed capillary tube is known to melt with decomposition at 323–325°C. Dispersion stability. Particle precipitate obtained in DMF was hydrophilic, readily dispersible to form col loidal solutions in water and polar solvents (DMF, acetone, ethanol) and chloroform. Particles in disper sions were initially aggregated in a certain extent. According to PCS, the hydrodynamic diameters of nanoparticle aggregates in initial dispersions based on water and DMF were ~80–90 nm, while those for dis persions based on chloroform were ~150 nm. Disper

sion persistence toward particle coagulation and sedi mentation also depended on solvent. Figure 6 shows time variations of spectra, wavelength maxima of SPR bands, and optical density of dispersions, as well as average hydrodynamic diameter of nanoparticle aggregates in these solutions; zero time reference cor responds to the instant of dispersion preparation. In water, showing the highest solvating ability, solu tion was stable at relatively low concentration cAu = 0.15 mM for the first 2 hours: spectrum showed one main SPR band, its parameters λmax = 541 nm and absorption at maximum A ≈ 0.63 remained constant, average size of nanoparticle aggregates (dha) was ~95 nm. Coagulation interactions of particles increased in sub sequent 2 hours, absorption in the main band region slightly decreased, its λmax was shifted to red region to 547 nm due to appearance and growth of absorption in the region λ > 600 nm. Isosbestic point was observed at ~580 nm, while aggregate size increased to dha ≈ 102 nm. Sedimentation of particles as aggregates with dha > 100 nm began approximately after 24 h. The isosbestic point


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A 1–16

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λmax, nm

550 (а)

(b)

545 540

17

535

18

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A

19 0.5

0.3 0.3 dha, nm

0 200

500 λ, nm

(d)

100 80 60

800

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0

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A 1.2

A

1 2–7 8 9 10

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100 50

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Fig. 6. Dynamics of changes in spectral and dimensional characteristics of nanoparticle aggregates in dispersions based on water, DMF, and chloroform. (a–j) Disperse phase is nanoparticle precipitate isolated in the synthesis in DMF. Dispersion media: (a–d) water, cAu = 0.15 mM, l = 0.5 cm, λmax = 541 nm, t = 20– 245 (1–16), 1527 (17), 2846 (18), 4547 min (19); (e–g) DMF, cAu = 0.43 mM, l = 0.5 cm, λmax = 551 nm, t = 5 (1), 113–310 (2–7), 1480 (8), 7300 (9), 10143 min (10); (h–j) chloroform, cAu = 0.33 mM, l = 0.5 cm, λmax = 559 nm, t = 12 (1), 117 (2), 312 (3), 1505 (4), 4385 min (5). (k–m) Disperse phase is nano particle concentrate prepared by synthesis in water, dispersion medium is water, cAu = 0.16 mM, l = 1 cm, t = 12 (1), 69 (2), 135 (3), 227 (4), 304 (5), 1256 (6), 3006 min (7).

A

(k)

(l)

1.8

A

2 1–7

1.3 0.8

1 (m) dha, nm

200

0 250

500 λ, nm

750

150 100 10

100

1000 t, min

Fig. 6. Contd.

disappeared in the course of sedimentation, intensity of all spectrum gradually decreased, which indicated decrease in gold concentration in solution. Because of sedimentation of large aggregates and decrease in gold concentration, coagulation interactions became weaker and the main SPR band returned to the initial position λmax = 540 nm, while the average hydrody namic diameter of aggregates was stabilized at the level dha ≈ 71 nm after about 45 h. Coagulation interactions were more distinct in DMF at concentration cAu = 0.43 mM. Two hours after preparation, two SPR bands appeared in disper sion spectrum with maxima at 552 nm, whose position did not change, and at 710–720 nm. Like in aqueous solution, sedimentation began after about 1 day. In the course of monitoring, solution absorption at λmax = 552 nm monotonically decreased initially due to redis tribution of optical density between the bands and then on account of sedimentation, while aggregate size mono tonically increased in time until sedimentation beginning and then it stabilized about dha ≈ 159 nm. In the least polar chloroform, the initial size of aggregates was large, therefore sedimentation began immediately after dispersion preparation. The average size of aggregates dha ≈ 150–160 nm and the shape of spectrum with the main SPR band at λmax > 600 nm remained constant, while the intensity of all spectrum monotonically decreased. It was found by nonaqueous

electrophoresis that nanoparticle aggregates had positive charge, electrokinetic potential ζ = 102 ± 6 mV, and elec trophoretic mobility μ = (51 ± 6) × 10–10 m2/(V s), errors are given as confidence intervals for Р = 0.95. When particles obtained in aqueous solution and isolated as dry concentrate were redispersed, they showed spectrum with the main SPR band at λmax = 535 nm, which was shifted by 17 nm to the red region as compared with the band for particles in solution in situ on synthesis at λmax = 518 nm. Moreover, the ini tial diameter of aggregates dha ≈ 200–210 nm was larger than critical size ~100–160 nm, therefore sedi mentation began immediately after dispersion prepa ration. In the course of the process, spectrum intensity decreased uniformly, at the same time, the position of the main band remained constant at λmax = 535 nm. After precipitation of large aggregates formed because of drying of nanoparticles upon their isolation as con centrate, the average size of remained aggregates decreased to dha ≈ 110 nm. Thus, our study showed that isonicotinic acid can behave as a protective ligand in the synthesis of gold nanoparticles in water, DMF, and DMSO. The results of synthesis are dependent on the absolute values and ratio of concentrations of initial reagents: HAuCl4, INA, and NaBH4, as well as the solvent. To obtain col loidal solutions of nonaggregated nanoparticles in all solvents, optimal conditions are relatively small gold


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concentration cAu ≤ 0.5 mM and maintenance of con centration ratio HAuCl4 : INA : NaBH4 = 1 : 2 : (5–10). Reaction products in water are mainly primary parti cles having a spherical shape and gold nuclei diameter within 2–7 nm, products in DMF include secondary particles that constitute agglomerates of primary par ticles of spherical or slightly faceted shape 2–12 nm in diameter as well as larger extended agglomerates 15– 100 nm long. At ratio larger than HAuCl4 : INA = 1 : 2, agglomeration increases and leads to preparation of chaotic scaffold nanostructures that may be of interest for catalysis. The products of synthesis of gold nano particles isolated as precipitate from DMF and as con centrate from water readily produce colloidal solu tions upon redispersing in water and polar solvents. Nanoparticles in dispersions are in aggregated state and suffer gradual coagulation and sedimentation. In dispersions based on DMF and water, sedimentation begins one day after solution preparation and is not irreversible, the disperse state can be restored by ultra sound treatment. Nanoparticle precipitate obtained in DMF contains along with gold and INA anion also acetone, water, sodium, and oxygen boron com pounds. These data confirmed our previous observa tion that acetone used for precipitate washing as well as sodium ions and hydrated boron oxides resulting from NaBH4 decomposition are inclined to incorpo rate into composition of gold nanoparticle precipitates synthesized by the reduction of HAuCl4 with sodium borohydride. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (project no. 120300091a). REFERENCES 1. Metal Nanoparticles and Nanoalloys, Ed. by R. L. Johnston and J. P. Wilcoxon (Elsevier, Amster dam, 2012), Vol. 3. 2. N. Krasteva, I. Besnard, B. Guse, et al., Nano Lett. 2, 551 (2002). 3. R. W. Gerber, N. L. Donovan, and S. Franzen, Thin Solid Films 517, 6803 (2009).


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Translated by I. Kudryavtsev

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2015