Sensitivity of the Multiple Functional Moieties of Amino Acids for the Self- Assembly of Au Nanoparticles on Different Physicochemical Properties

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Journal of Cluster ScienceIncluding Nanoclusters andNanoparticles ISSN 1040-7278Volume 25Number 4 J Clust Sci (2014) 25:1085-1098DOI 10.1007/s10876-014-0691-7

Sensitivity of the Multiple FunctionalMoieties of Amino Acids for the Self-Assembly of Au Nanoparticles on DifferentPhysicochemical Properties

Anila Monga & Bonamali Pal

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ORI GIN AL PA PER

Sensitivity of the Multiple Functional Moietiesof Amino Acids for the Self-Assembly of AuNanoparticles on Different Physicochemical Properties

Anila Monga • Bonamali Pal

Received: 24 August 2013 / Published online: 23 January 2014

� Springer Science+Business Media New York 2014

Abstract This paper investigates the extent of the self-assembly process of Au

nanoparticles, depending on the nature of structural and functional moieties of

various amino acids (L-cystine, glutathione, L-cysteine and N-acetyl cysteine) and

their influence on the plasmon sensitivity and electrokinetic parameters in corre-

lation with the catalysis of p-nitrophenol reduction. DLS particle size analysis

revealed that the hydrodynamic size 10–20 nm of Au nanospheres was increased to

135–550 nm, 100–460 nm and 130–240 nm after the addition of L-cystine, L-cys-

teine and glutathione, respectively, in contrast to no significant change of particle

size (15–60 nm) after N-acetyl cysteine addition. This difference in the extent of

aggregation as a function of structures of amino acids is further evidenced by

lengthy tubular arrays formation by glutathione as compared to branched chain like

morphology obtained by L-cystine through TEM. FTIR studies further confirmed the

binding of amino acids to Au nanospheres via –SH followed by linking of adjacent

nanoparticles through H-bonding. Due to the conformational diversity of amino

acids, the surface adsorbed –SH, –COO- and –NH3? species over assembled Au

nanoparticles led to the alteration of zeta potential and conductance, thus affected

the catalysis for the reduction of p-nitrophenol as compared to unmodified Au

nanoparticles.

Keywords Self-assemble Au nanospheres � Amino acid modified

Au nanoparticles � Electrokinetic parameters � Catalytic activity

Electronic supplementary material The online version of this article (doi:10.1007/s10876-014-0691-7)

contains supplementary material, which is available to authorized users.

A. Monga � B. Pal (&)

School of Chemistry and Biochemistry, Thapar University, Patiala 147004, Punjab, India

e-mail: [email protected]

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J Clust Sci (2014) 25:1085–1098

DOI 10.1007/s10876-014-0691-7

Author's personal copy

http://dx.doi.org/10.1007/s10876-014-0691-7

Introduction

Self-assembly of nanoscale particles is the most promising route to create new

macroscopic hybrid materials which exhibit unique functionality. It is a reversible

process and has beneficial advantages [1–5] over random aggregation as it results in

lower Gibbs’s free energy [5, 6] and hence thermodynamically stable assembled-

structures than non-assembled structures. The close proximity of two or more

particles in assembled nanostructures gives rise to interaction of conduction electron

oscillations of each nanoparticle (NP) and displays rich optical, catalytic and

electrical characteristics that are distinctly different from individual particles [7–

14]. The interactions involved usually are non-covalent, such as electrostatic

interactions, hydrogen bonds, Vander Waals’ forces, coordination interactions and

solvophobic effects [15–17]. A variety of structures have been obtained by the

assembly process, including chains of nanorods (NRs), nanonecklaces, and

alternating bipyramid-nanosphere (NS) chains [18–24] for various applications.

AuNSs were organized into linear aggregates by ethanol due to dipole–dipole

interaction in the solution [25]. Sen and Patra [26] studied the optical properties of

chain-like assembled AuNPs using 3-mercaptopropionic acid and 2-mercap-

toethanol while Mirkin et al. [27] exploited the AuNPs assembly for colorimetric

sensing and sequencing strategies for DNA.

The AuNPs have high affinity for functional groups such as –SH, –COOH and

–NH2 present in amino acids (e.g. L-cystine and glutathione) where the coupling

arises from thiol (–SH) attachment to the NPs followed by cross-linking via two-

point electrostatic interactions of the exposed zwitterion functionalities (–COO-

and –NH3?) [18, 19]. These positively charged amines and negatively charged

carboxylates species on NPs surface imbalances the uniform charge present on Au

nanostructures suspension and develop a potential difference at the interface, which

is measured in terms of zeta potential (n) responsible for the stability of a colloidal

system [28, 29]. For instance, Kim et al. [29] observed that the zeta potential of

AuNSs (-52.72 mV) in water was reduced (-41.38 mV) by capping with benzyl

mercaptan. The reduction in n from ?47.6 ± 3.3 to ?10.9 ± 4.1 mV was observed

during addition of 10-5 M adipic acid in aqueous suspensions of AuNRs [30] due to

the electrostatic interactions between them. The ordered agglomeration is strongly

dependent upon the chemical structure of linking agents, for example; L-cystine is a

dimeric amino acid having disulfide and a pair of zwitterion groups while

glutathione is a tripeptide with one –SH and one zwitterion group as shown in

Scheme 1. This charge alteration of AuNPs after the assembly by these ionic species

affects the adsorption of reacting substrates over nanocatalyst’s surface, thereby

control the catalytic activity. Hence, this work demonstrates the nature of AuNSs

aggregation due to surface passivation with L-cystine and glutathione and

investigates their optical, electrokinetic and catalytic properties as compared to

unmodified AuNSs.

1086 A. Monga, B. Pal

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Experimental Details

Materials

Chloroauric acid (HAuCl4�3H2O), sodium borohydride (NaBH4), ascorbic acid

(C6H8O6), glutathione (C10H17N3O6S), L-cystine (C6H12N2O4S2), L-cysteine

(C3H7NO2S), N-acetyl cysteine (C5H9NO3S) and p-nitrophenol (C6H5NO2) were

obtained from Loba Chemie, India. Silver nitrate (AgNO3), and cetyltrimethylam-

moniumbromide (CTAB, C19H42BrN) were purchased from Fischer Scientific and

Sigma Aldrich, respectively. All the chemicals were used as-received without any

further purification. De-ionized water was obtained using an ultra-filtration system

(Milli-Q, Millipore) with a measured conductivity above 35 mho cm-1 at 25 �C.

Synthesis, Self-Assembly, Characterization and Catalytic Activity of AuNPs

The AuNSs were synthesized by a seed-mediated approach in aqueous media as

reported elsewhere [31, 32]. An aqueous solution (20 ml) containing HAuCl4�3H2O

(500 ll, 0.01 M) and trisodium citrate (1.47 mg, 2.5 9 10-4 M) was prepared

followed by reduction with NaBH4 (600 ll, 0.1 M) solution under magnetic stirring

for 2 min and resulted in the formation of Au seeds. This seed solution (ca. 2.5 ml)

was introduced into the mixture of HAuCl4�3H2O (500 ll, 0.01 M), CTAB (9.5 ml,

0.08 M) and ascorbic acid (50 ll, 0.1 M). The red-colored solution showing the

surface plasmon (SP) band at 529 nm indicated the formation of spherical AuNPs.

Whereas for the synthesis of AuNRs, this seed solution (ca. 500 ll) was added into

25 ml aqueous solution mixture containing HAuCl4�3H2O (1.25 ml, 0.01 M),

CTAB (920 mg), AgNO3 (225 ll, 5 mM) and ascorbic acid (200 ll, 0.1 M). The

deep-blue color solution showing transverse surface plasmon (TSP) band at 536 nm

and longitudinal surface plasmon (LSP) band at 743 nm indicated the formation of

AuNRs. The AuNPs were washed with de-ionized water under centrifugation at

8,500 rpm for 10 min. After recording the UV–Vis absorption spectrum of a 2 ml

Scheme 1 Schematic representation of linked AuNSs by amino acids

Sensitivity of the Multiple Functional Moieties 1087

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(6.72 9 1017 atoms, see electronic supplementary info) of prepared AuNPs in

quartz cell, a certain amount of (100 ll, 100 mM) L-cystine (L-cys), glutathione

(glut), L-cysteine (cyt) and N-acetyl cysteine (N-acyt) were added into the AuNPs

solution and SP band absorption was recorded after a regular interval of time. The

obtained Au nanostructures capped with various amino acids were characterized by

UV–Vis absorption (Analytic Jena specord-205) spectrophotometer and TEM

(Hitachi 7500, 2 A, 120 kV). Fourier transform infrared (FTIR) spectra of pure

amino acid (i.e., glut) and glut–AuNS was recorded with Agilent Cary 630 FTIR

spectrometer. The solution containing 2 ml AuNSs (6.72 9 1017 atoms) and 100 ll

of amino acids (100 mM) were taken in a cuvette for the zeta potential and DLS

measurements by Brookhaven 7610 instruments. The catalytic reduction of

p-nitrophenol (PNP) was carried out by adding an ice-cold NaBH4 solution

(500 ll, 0.42 M) to PNP (5 ml, 0.2 mM), and stirred for 5 min at room temperature.

Then, the calculated amount of AuNSs [20 ll, 6.72 9 1015 atoms, ESI-(1)] was

added to initiate PNP reduction to the p-aminophenol (PAP) formation by

measuring the absorption spectra (kmax of PNP * 400 nm and PAP * 300 nm)

at regular intervals of time. The GC–MS analysis of reduction products was

conducted with Bruker GC-45X with Scion MS system equipped with RTX-5 MS

Sil column (15 m 9 0.25 mm 9 0.25 mm) and NMR spectrum was also taken with

Bruker Avance-II with a frequency of 400 MHz for 1H detection.

Results and Discussions

Optical Properties of Amino Acid Modified Au Nanostructures

The effect of different amount (20–100 ll) of L-cys (100 mM) addition into an

aqueous suspension of AuNSs and AuNRs was studied at a regular interval of time.

The intensity of the SP absorption band of AuNSs at 529 nm is gradually decreased

with increasing amount of L-cys and is red-shifted to the longer wavelength

(625 nm) at higher concentration of L-cys (100 ll) as seen in Fig. 1. Similarly, in

AuNRs, the LSP band is red-shifted from 743 to 780 nm with decreased intensity on

adding a higher amount of L-cys, whereas the transverse band with reduced

absorption intensity is remained at 526 nm as shown in ESI-Fig. 1. The kinetics of

AuNSs and AuNRs surface passivation with the different amount of L-cys

(100 mM) clearly revealed (Fig. 2) that low concentration (20–60 ll) of L-cys

addition does not show any significant change in SP band except a linear decrease in

its intensity; however, beyond 80–100 ll, there is a remarkably bathochromic shift.

The variation in SP band absorbance and red-shifting arises in opposite trend

probably due to the electromagnetic coupling of SP electrons of adjacent AuNPs

linked by –SH, –COOH and –NH2 moieties of L-cys. As a result, the AuNPs exhibit

increased scattering and broadened SP peaks towards longer wavelengths.

Figure 3 shows the time course assembly of AuNSs with a fixed amount (100 ll,

100 mM) of L-cys, where the intensity of SP band (529 nm) is gradually reduced

with the simultaneous evolution of a new flat band at 643 nm, which shifted to

724 nm after 20 min of L-cys addition. The former band corresponds to the


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transverse SP band, while the latter one is accepted as longitudinal SP resonance

coupling contributed from the oscillations of electrons along the axial direction of

the anisotropic Au nanostructures [33]. On the other hand, glut addition (100 ll,

100 mM) led to the reduced intensity of the SP band (529 nm) with the red-shift at

585 nm only, after 40 min as noted in ESI-Fig. 2a, while the SP band of AuNSs

(529 nm) immediately red-shifted to 581 nm and finally became constant at 635 nm

of reduced intensity, after 10–15 min of addition of cyt. In contrast, the addition of

N-acyt does not influence the plasmon absorption of AuNSs as seen in ESI-Fig. 2b.

Fig. 1 UV–Vis absorption spectra showing SP band of AuNSs as a function of various amounts ofL-cystine (100 mM)

Fig. 2 Variation of absorbance and SP band wavelength with the different amount of L-cystine fora AuNSs, and b AuNRs


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Such changes in the SP band of L-cys–AuNS, glut–AuNS and cyt–AuNS reveals the

formation of aggregated AuNSs as evidenced from the observed color change (inset,

Fig. 3, ESI-Fig. 2a) from deep red to dark blue.

Figure 4 shows a noted difference in the change of absorption intensity and SP

band wavelength during AuNSs aggregation with the certain amount of different

amino acids i.e., L-cys, glut and cyt, where the absorbance at 529 nm displays a

gradual decrease and increase in absorbance at 724 nm with time in the case of

L-cys–AuNS. This is in good conformity with the reported red-shifting of the SP

band from 521 to 680 and 735 nm by 2-mercapoethanol and 3-mercaptoproponic

acid modification, respectively [26]. Similarly, the SP band in cyt modified AuNSs

tends to red-shift (635 nm) immediately with a simultaneous decrease in intensity.

However, in glut–AuNSs, the change in SP band was less appreciable and slow,

suggesting the self-ordered AuNSs assembly occurs in a different extent as depicted

in Scheme 1. On the contrary, N-acyt does not affect the SP absorption band of

AuNSs indicating the absence of any kind of such aggregation. It is reported that the

AuNPs in various polar solvents show a characteristic red-shifted SP band peak at

*974 nm [34] due to random agglomeration owing to dipole–dipole interaction and

the absence of such a long-wavelength SP band in the case of L-cys/glut–AuNS

morphology, suggesting different conformation of AuNSs aggregation, as further

confirmed by DLS and TEM size analysis.

DLS and TEM Morphology of Amino Acid Modified Au Nanostructures

DLS particle size analysis showed that the hydrodynamic diameter in the range of

8–50 nm (average size = 20 nm) of bare AuNSs (Fig. 5a) is appreciably increased

Fig. 3 UV–Vis absorption spectra of bare AuNSs after the addition of the fixed amount (100 ll) of100 mM L-cystine


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to 100–460 nm (average size = 225 nm) and 130–240 nm (average

size = 182 nm) in case of cyt–AuNS and glut–AuNS assembly, respectively,

which is much lower than the agglomerated size 135–550 nm (average

size = 278 nm) for L-cys–AuNS as shown in Fig. 5b. As it is proposed that both

the thiol and the zwitterion forms are essential for the assembly of AuNPs [18], and

the presence of two thiolate and two zwitterion groups in L-cys increases the

chances of linkage than cyt and glut having only one –SH and one zwitterion group

(–COO- and –NH3?), are probably responsible for the larger extent of aggregation

in case of L-cys–AuNS resulting in bigger hydrodynamic diameter. Particle size

distribution of N-acyt modified AuNSs is observed to be in the range of 15–60 nm

equivalent to bare AuNSs as compared to larger particle size distribution exhibited

by other amino acids assembled nanostructures. This can be attributed to the lack of

electrostatic interaction of zwitterionic functionalities (–COO- and –NH3?) due to

the absence of free –NH2 moiety in the structure of N-acyt, which confirms the role

of the zwitterionic group in self-assembly process.

TEM morphology (Fig. 6a) showed that the well-dispersed AuNSs (size

8–15 nm) are typically arranged into two parallel linear chains at a distance of

*80 nm after glut addition making end to end close contact with hollow tubular

morphology as seen in Fig. 6b–e. Some distortion of the spherical shape of

agglomerated AuNSs is visible in Fig. 6c, d as also reported by Wang et al. [35]. In

some places, significant branching of the assemblies occurred in-between the

tubular chains of glut–AuNSs nanostructure (Fig. 6e) giving a ladder-like shape.

The interaction of NSs occurred towards longitudinal direction and thus creates a

curved shape lengthy morphology assembles of AuNSs enclosed in a cross-sectional

diameter of *40 nm (Fig. 6f, g). On the other hand, L-cys–AuNSs are arranged like

grape bunches with deformed surface as observed in Fig. 7a, b. Many AuNSs are

orderly organized into a strand similar to the necklace like association (Fig. 7c, d)

Fig. 4 Time course graph of absorbance and SP band wavelength for assembled AuNSs by L-cystine,glutathione and L-cysteine


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that are almost similar to chain-like [7, 36] Au nanostructures arrays. These results

are in agreement with the linear assembly of AuNRs in an end to end fashion, which

is never observed in the absence of cysteine and glutathione [19] as it is reported

that the presence of the –SH group, the amino and carboxyl groups in amino acid

molecules is the key to the well-ordered end-to-end assembly [18, 19]. Mandal et al.

[37] also demonstrated the importance of hydrogen-bond for fabrication of 2D web-

like fused Au nanostructures which cannot be achieved by non-hydrogen-bonding

surfactant. Similar morphology for assembled-AuNSs by using mercaptoethanol and

mercaptopropionic acid [26] and polar solvents like methanol, ethanol, propanol etc.

[38] is also reported. Here, the disulfide bond of L-cys is broken close to the surface

of NPs with the generation of two thiolates, which are then selectively

functionalized onto AuNSs [39], leaving two zwitterion groups for further

electrostatic interaction with appended zwitterion groups of adjacent NPs assisting

the coupling. Hence, the subsequent fusion of the assembled particles in L-cys–

AuNS is possibly due to the strong electrostatic linking of adjacent particles via two

–S- and two zwitterion groups, which are one in case of glut. These results confirm

the importance [19] of thiols, amines and carboxylic acid groups in the self-

assembly of AuNPs.

FTIR Study

The surface binding of amino acids (like glutathione) onto AuNSs is confirmed by

the FTIR measurements. The prominent band centered at about 2,527 cm-1

corresponding to the S–H vibrational band is clearly seen for the free glutathione

molecule [curve (i), ESI-Fig. 5], which disappeared after the addition of AuNSs

[curve (ii), Fig. 5]. This provides strong evidence that the surface binding of

glutathione to AuNSs occurs through the Au–S linkage as depicted in Scheme 1.

The carboxylate stretch vibration of the glutathione molecule is observed to occur at

1,725 cm-1 and is shifted to 1,628 cm-1 in the presence of AuNSs. It reveals some

interaction of the acid group with other glutathione molecule located on adjacent

Fig. 5 DLS particle size distribution of a bare AuNSs, and b L-cys–AuNS, glut–AuNS and cyt–AuNS


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Fig. 6 TEM images of a bare AuNSs, and b–g AuNSs in the presence of 100 mM glutathione


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AuNSs, possibly through intermolecular H-bonding. These results are in agreement

with the previous report on L-cysteine modified AuNPs by Naka et al. [40].

Electrokinetic Parameters

When a NP is dispersed in an aqueous solution, there is a surface ionization or

adsorption of cations or anions, which results in the formation of electrical double

layer leading to the development of new surface charge measured by Zeta potential

(f) [41]. It is found here that the measured zeta potential ?22.85 mV (pH 6.7) for

bare AuNSs increased to ?26.11 mV for cyt–AuNS (pH 6.0), ?28.67 mV (pH 5.6)

for glut–AuNS and ?33.5 mV (pH 5.0) for L-cys–AuNS after the assembly process

as shown in Fig. 8a. This fact indicated that the charged species (COO- and NH3?)

in L-cys, glut and cyt aqueous solution having zeta potential ?9.5 (pH 2.7),

?2.78 mV (pH 3.9) and ?3.76 mV (pH 3.2), respectively, altered the net charge on

AuNSs. These higher zeta potential values revealed that the L-cys–AuNS, glut–

AuNS and cyt–AuNS nanostructures system are stable than the aqueous AuNSs

suspension as the assembly process leads to lowering of entropy due to ordered

arrangement and hence lowering the free energy. In contrast, the conventional

Fig. 7 TEM images of AuNSs in the presence of 100 mM L-cystine


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random aggregation process generally leads to the destabilization of colloidal

system reducing the zeta potential values [29, 30, 33]. Similar electrokinetic

properties are also observed; for example, zeta potential in the range of -29 to

-50 mV for citrate coated AuNPs [26, 42] and ?49 to ?71 mV for CTAB-coated

Au nanocrystals [6] depending on NP shape and size. As the assembly of AuNPs

takes place through thiolate linkage followed by cross-linking via two-point

electrostatic interactions of the exposed zwitterion functionalities (–COO- and

–NH3?) producing electrolytic current flow, while no such charge carriers are

present in bare ones. As a result, the conductance of L-cys–AuNS (22,728 lS), glut–

AuNS (5,902 lS) and cyt–AuNS (5,597 lS) was found to be higher than bare

AuNSs (2,323 lS) as seen in Fig. 8b because L-cys–AuNS contain more charged

species than glut as indicated in Scheme 1.

Impact of Aggregated Nanostructures on Catalysis

The observed difference in the degree of AuNPs aggregation from L-cys and glut are

further reflected in their catalysis property, which is very sensitive to surface

structure and particle size. Figure 9 (inset) shows the absorption spectra for

reduction of the PNP to PAP by NaBH4 in the absence of Au nanostructures, and no

reduction took place up to 120 min reaction. However, with the addition of AuNSs

(size = 8–10 nm, ca. 6.72 9 1015 atoms) to this mixture containing PNP (0.2 mM)

and excess amount of NaBH4 (0.42 M), the reaction rate is accelerated (Fig. 9) and

accordingly, the reduction rates can be regarded as being independent of the

concentration of NaBH4 [43]. Therefore, the presence of AuNPs catalyst is very

essential for the PNP reduction to occur as noted in the color changes in the inset of

Fig. 9 [44]. On addition of NaBH4 to PNP, the alkalinity of solution increased to

form p-nitrophenolate ions having a strong absorption band at 400 nm whose

intensity is gradually reduced with the simultaneous evolution of a new absorption

peak at 300 nm, characteristic of the PAP formation (ESI-Fig. 6). The difference in

the PNP reduction rate by the bare, L-cys and glut modified-AuNSs indicated that

Fig. 8 Electrokinetic parameters showing a Zeta potential, and b conductance of different Aunanostructures


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the bare AuNSs showed the highest conversion of PNP (95 %) as compared to

L-cys–AuNS (80.3 %) and glut–AuNS (80.6 %) morphology as shown in Fig. 10a.

The higher catalytic activity of bare AuNSs can be ascribed on the basis of free

surface accessibility for better electron transport from AuNSs to adsorbed PNP

molecules. However, aggregated L-cys–AuNS and glut–AuNS particles create

hindrance for electron transfer in PNP reduction due to partial blockage or access on

the AuNSs surface by the ionic species (–COO- and –NH3?) and decrease in

effective surface area, resulting in a considerable lower yield (36 and 47 %) of PAP

formation, respectively, as compared to bare AuNSs (86 % yield) during 60–75 min

reaction as shown in Fig. 10b. GC–MS analysis qualitatively evidences the

formation of p-aminophenol (m = 109) from p-nitrophenol (m = 138.9) (ESI-

Fig. 8c, f) corresponding to the GC-peaks for retention time (Rt) 12.0 and 16.1 min,

labeled as 3 and 6 in ESI-Fig. 7a, respectively. In addition to above mentioned

peaks, some other peaks at Rt = 10.8 min (o-aminophenol), 11.8 min (m-amino-

phenol) and 15.0 min (o-nitrophenol) and 15.2 min (m-nitrophenol), designated as

1, 2, 4 and 5, respectively in ESI-Fig. 7a are also found. This is because the reactant

used in the present study initially contains the traces of ortho (Rt = 15.0 min) and

meta-nitrophenols (Rt = 15.2 min) as seen in ESI-Fig. 7b, that reduces to their

respective amines as confirmed from their mass spectra (ESI-Fig. 8a, b). The

formation of reduced product p-aminophenol was also confirmed from the 1H NMR

spectra of the product (in d6-acetone, ESI-Fig. 9) which consists of four aromatic

protons as multiplet at d 6.5 and a singlet at d 8.24 due to O–H proton. The –NH2

proton appears at 3.0 as a singlet and the other signal at d 2.0 is due to the solvent.

Fig. 9 Change in absorption spectra of PNP (0.2 mM) reduction by NaBH4 in the presence AuNSs andinset shows the insignificant change in absorbance of PNP only in the presence of NaBH4 over a period of120 min


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Therefore, the self-aggregation reduces the catalytic activity for the reduction of

PNP as reported by Azetsu et al. [45] in which the aggregated AuNPs formed in the

absence of polymer (TOCNs) showed very poor catalytic activity in contrast to

polymer stabilized NPs. It was also found that the AuNPs aggregates are

catalytically inactive for the hydrosilylation of 4-methoxybenzaldehyde due to the

reduced solvent-exposed surface area [46]. Thus, the degree of AuNSs coagulation

is judiciously reflected in the catalytic effect relative to bare quantum size AuNSs

catalyst.

Conclusions

In summary, we have demonstrated the role of linking moieties required for the self-

assembly of AuNPs using various structural amino acids. The presence of –SH

group, the amino and the carboxyl groups in the amino acid molecules is the key to

end-to-end assembly. The difference in geometrical conformation of L-cys, glut and

cyt with multiple surface active moieties led to different extent of the assembly

process. In contrast, N-acyt does not lead to the assembly of AuNPs due to absence

electrostatic interactions in zwitterionic group. These ordered assemblies by amino

acids are found to be relatively different than pH triggered or dipolar agglomeration

in polar solvents as evident by the observed changes in TEM. Due to a reduction in

the effective surface area of coagulated AuNPs having pre-adsorbed ionic species

create hindrance in electron transfer and adsorption of reacting substrate and thus,

regulate the catalysis process depending on their surface structural changes occurred

by the attached amino acids’ functionality.

Acknowledgments We acknowledge Dr. B. K. Chudasama (School of Physics and Material Science,

Thapar University) for Zeta potential, conductance and DLS measurement. We would also like to thank to

Sophisticated Analytical Instrumentation Facility (IIT Bombay) for TEM analysis.

Fig. 10 Time course graph showing the change in concentration (lmol) of a PNP remained, and b PAPproduced by the influence of different Au nanostructures (AuNSs, L-cys–AuNS, glut–AuNS)


123


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