New Photonic Materials based on Ag Nanoparticles Modified with

Stilbene Dyes and Its Peculiar Behavior Studied with SERS

Alexey N. Smirnov, Olga V. Odintsova and Elena V. Solovyeva

Chemistry Institute, Saint-Petersburg State University, 26 Universitetsky pr., Peterhof, Saint-Petersburg, Russia

Keywords: SERS, Raman, Stilbenes, Amines, Hot Spots, Ag Nanoparticles.

Abstract: In the recent years, there has been a growing interest in development of smart photoactive materials with

variable properties. This work presents the SERS study of behaviour of organometallic composites obtained

by modification of Ag nanoparticles in hydrosols with several amino derivatives of stilbene. The feasibility

of bifunctional stilbenes to be the molecular linkers of Ag nanoparticles is discussed. The hot spots activity

of such modified Ag nanoparticles is considered. Two exciting but not fully understood finding were made:

i) tertiary amines can be used as the effective molecular linkers; ii) the structure of central fragment between

two benzyl rings has a strong influence on the modifier ability to incorporate the Ag nanoparticles into

agglomerates with hot spots. The limitations consisting in the pH value and the presence of chloride ions are

described for a potential application of the developed hot spots substrates.

1 INTRODUCTION

Development of new photonic materials is a subject

of great interest in modern physics, chemistry and

materials science. Integration of photoactive

molecules with plasmonic nanostructures represents

one of the most promising approach for engineering

such materials. Among the plasmonic

nanostructures, the substrates with hot spots are

particularly attractive. Hot spots are the regions

between closely located nanoroughnesses where a

superposition of local electromagnetic fields occurs

(Schlücker, 2014; Roelli 2016). Inside the hot spots

greater enhancement of optical signal, e.g. Raman

scattering, is provided. Hot spots can be obtained by

self-assembling nanoparticles using molecular

linkers. The term “molecular linkers” denotes the

molecules which have a capability of simultaneous

attachment to two different surfaces. Due to higher

optical response from hot spots, among the

photoactive molecules able to electrostatically or

chemically interact with a substrate the choice is

preferable for bifunctional ones.

In this study, we present the investigation of

organometallic composites based on Ag

nanoparticles (Ag NP) modified with

aminostilbenes. This class of organic pigments has

the pronounced photoactivity (Su, 2017; Liu, 2000).

A combination of aminostilbenes photophysical

properties with the plasmonic properties of Ag NP is

promising strategy for obtaining the systems whose

optical properties can be controlled by external

forces. In our previous work, we focused on 4,4'-

diaminostilbene and its ability to link the metal

nanoparticles was demonstrated (Solovyeva, 2018).

In the present study, the particular attention was paid

to exploring an influence of dye’s structure on the

molecule-surface interaction. Three dyes differing in

the functional groups and saturation of central bond

(see Fig. 1) were investigated in silver hydrosols by

surface enhanced Raman spectroscopy (SERS) and

transmission electron microscopy (TEM). It was also

importantly to evaluate a behavior of obtained

systems under changing conditions. Such parameters

as pH or presence of chloride anions may be crucial

for electric double layer structure and Ag NP –

molecule interaction consequently.

Figure 1: Common chemical structure of investigated

compounds. 1a – DAS; 1b – ADMAS; 1c – AS, 2 –

DABB (meaning of abbreviations see in Experimental).

Smirnov, A., Odintsova, O. and Solovyeva, E.

New Photonic Materials based on Ag Nanoparticles Modiﬁed with Stilbene Dyes and Its Peculiar Behavior Studied with SERS.

DOI: 10.5220/0007569302630267

In Proceedings of the 7th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2019), pages 263-267

ISBN: 978-989-758-364-3

263

2 EXPERIMENTAL

4-aminostilbene (AS) (98%), 4,4'-diaminostilbene

dihydrocloride (DAS) (95%), 4-amino-4'-(N,N-

dimethylamino)stilbene (ADMAS) (98%) and 4,4'-

diaminobibenzyl (DABB) (95%), were purchased

from Sigma-Aldrich. DAS was recrystallized in

MeOH before using. Other compounds were used

without further purification. Methanol solutions of

AS and ADMAS with concentration in the range

from 1×10

-7

to 1×10

-5

M were prepared from stock

solution (1×10

-4

M) by volume dilution method for

SERS measurements. Silver hydrosol was prepared

by reduction of silver nitrate by sodium borohydride

in accordance with the standard procedure. To

decrease pH of solution, nitric acid with appropriate

concentration was used. To provide the halide

anions effect, potassium chloride was added in silver

hydrosol.

The SERS spectra were recorded using LabRam

HR800 (Horiba Jobin Yvon) spectrometer with CCD

detector. The incident laser excitation was 488 nm

line from Ar

laser source. Laser power at a sample

was 20 mW. All SERS spectra were registered in the

range of 400-1800 cm

-1

, in four acquisitions, 20 s

accumulations. The images of Ag nanoparticles were

obtained with a Zeiss Libra 200FE transmission

electron microscope (TEM) at an accelerating

voltage of 200 kV. TEM images in scanning mode

(STEM) were taken from at least three random

domains of the sample. In order to prepare samples

for TEM measurements, 10 µl of silver hydrosol

were drop casted on top of carbon films and air

dried. During solvent evaporation the films were

kept in a dark place.

3 RESULTS ANS DISCUSSION

A concentration of organic additives in some cases

has a crucial influence on the properties of metal

hydrosols. Therefore, a wide range of concentrations

has to be considered for firstly applied modifiers.

Two types of concentration dependence of SERS

spectra were found for the studied stilbene

derivatives. As one can see from Fig. 2, the

monotonous growth of SERS signal along with the

concentration is observed for AS. While the non-

monotonic change of SERS intensity together with

the significant transformation of spectral profile can

be seen for ADMAS. The growth of intensity

follows to the increase of ADMAS concentration up

to 1 × 10

-6

M. The drop of SERS signal is further

observed proceeding up to 1 × 10

-5

M. The same

type of SERS spectra dependence on concentration

was obtained for DAS in our previous study

(Solovyeva, 2017).

600 1200 1300 1400 1500 1600 1700

ADMAS

1E-7 5E-7 1E-6 5E-6 1E-5

Raman shift (cm

-1

)

Figure 2: Dependence of SERS spectra on the

concentration (M) of ADMAS and AS in silver hydrosol.

The monotonous increase of SERS intensity in

case of AS corresponds to conventional gradual

filling of surface-solution interface without a

substantial alteration of molecular layers

configuration and surface morphology. The behavior

of ADMAS spectra similar with DAS suggests that

the same adsorption phenomena produce the

observed spectral changes. When the first monolayer

becomes completed, the way of molecules

interaction with surface changed. Obviously, this

happens at 1 × 10

-6

M and, thus, produces the

observed intensity drop and transformations of

spectral profile. At sub-monolayer adsorption,

ADMAS, as a bifunctional molecular linker,

interacts simultaneously with two different

nanoparticles by donation of nitrogens lone pairs. In

such conditions, ADMAS molecules are located in

the hot spots and its Raman scattering undergoes to

higher enhancement. At multilayer adsorption,

linking the nanoparticles via ADMAS becomes

impossible that leads to the ordinary Raman

enhancement.

The proposed hypothesis is consistent with the

data obtained by scanning transmission electron

microscopy for the same systems (fig 3). In the

presence of AS at any concentration, the

nanoparticles size and morphology do not change

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

264

significantly in comparison with the pure silver

hydrosol. In case of ADMAS, the oblong

agglomerates, which look like the glued

nanoparticles, can be seen in the TEM images

obtained for hydrosol with 1 × 10

-6

M of modifier.

For solution with 1 × 10

-5

M of ADMAS, the TEM

showed the small spherical nanoparticles as are

typical for borohydride-reduced silver hydrosol.

Figure 3: TEM images of Ag NPs modified by AS and

ADMAS at different concentrations.

Thus, we can conclude that the methyl

substitutes in one of two amino groups of ADMAS

do not obstruct to its coordination with surface.

Thereby, ADMAS has the same linking ability as

DAS. Such results are quite surprising but they mean

that a list of compounds, which can be used as

molecular linkers, can be significantly expanded.

The corresponding adsorption models are depicted at

fig. 4.

The solutions of AS and ADMAS in silver

hydrosol were further investigated in various

environments to get a deeper insight into adsorption

1200 1400 1600

5000 a.u.

Raman shift (cm

-1

)

40000 a.u.

ADMAS

Figure 4: Schematic models of adsorption for mono- and

bifunctional stilbene dyes at 1 × 10

-6

M conc. at Ag

nanoparticles in hydrosol.

mechanisms and to evaluate the limits for practical

applications of modified Ag NPs. The SERS spectra

of stilbene derivatives at neutral and acidic pH are

presented as Fig. 5. For ADMAS the effect of

solution acidification is examined at various

concentrations corresponding to two different

adsorption modes. The most substantial influence of

pH is observed for ADMAS at concentration of 1 ×

-6

M, when the molecules are attached to the two

different nanoparticles. In this case, the intense

SERS spectrum transforms to the spectrum of lower

intensity, and the noticeable bands shifts can be seen

also. It is should be noted that the profile of

spectrum at 1 × 10

-6

M ADMAS after acid addition

converts to the profile similar with those observed

New Photonic Materials based on Ag Nanoparticles Modiﬁed with Stilbene Dyes and Its Peculiar Behavior Studied with SERS

265

for 1 × 10

-5

M ADMAS at pH 2. Apparently,

molecules of ADMAS become disabled to «glue»

the nanoparticles after protonation of amino

(dimethyl) group. Protonated ADMAS molecules

adsorb on the surface in the same way whether sub-

monolayer or multilayer coverage. The changes in

the spectral profile are caused by electron density

redistribution. This is highlighted by the fact the

most significant profile change is the intensity

increase of modes near 1369 cm

-1

relating to the

deformational vibrations of double bond (Solovyeva,

2019). Probably, a reorientation of molecules

induced by protonation also takes place.

600 1200 1300 1400 1500 1600 1700

Raman shift (cm

-1

)

ADMAS

1E-6 M

pH 7 pH 2

1E-5 M

ADMAS

1E-5 M

Figure 5: Dependence of Raman signal from pH for

ADMAS and AS 10

-6

and 10

-5

M solutions in Ag

hydrosol.

The halide anions have a high affinity to silver

surface. The study of organics in metal hydrosols in

their presence allows one to evaluate the processes

of competitive adsorption. To this end, a potassium

chloride was added to the prepared organometallic

systems. Fig. 6 represents the effect of chloride

anions on the SERS spectra of AS and ADMAS. As

can be seen, the dramatic changes of SERS spectrum

occur for ADMAS at low concentrations

corresponding to sub-monolayer adsorption. In case

of AS, as well as for ADMAS at multilayer

adsorption, the addition of chloride ions is followed

only by moderate increase of intensity and some

bands transformations in their spectra. Apparently,

chloride ions, embedding in the double electric

layer, displace the ADMAS molecules from the first

monolayer. This leads to the observation of ADMAS

spectra characteristic for multilayer adsorption when

the SERS signal is collected from several layers,

including the top layers. It is should be mentioned,

that the observations for ADMAS at protonation and

chloride addition are the same with those that were

revealed for DAS.

600 1200 1300 1400 1500 1600 1700

ADMAS

1E-6 M

ADMAS

1E-5 M

Raman shift (cm

-1

)

without KCl 4.31E-2KCl

1E-5 M

Figure 6: Change of Raman signal for AS and ADMAS

after addition of chloride ions in Ag hydrosol.

A short time ago, we have investigated an ability

to link the Ag nanoparticles for bifunctional DABB,

which distinguishes from the stilbene derivatives by

the structure of central fragment. In DABB, the

benzyl rings are bonded with each other by the

saturated ethane fragment instead of the double bond

in stilbenes (see fig. 1).

Unexpectedly, the SERS spectra of DABB have

demonstrated the monotonous dependence on the

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

266

Figure 7: Dependence of SERS spectra of DABB on

concentration (M) in silver hydrosol.

concentration similar with AS (fig. 7). This suggests

that DABB do not link Ag NPs despite of two amino

groups. No color change observed for the

corresponding solutions of DABB in silver hydrosol

also implies that the Ag nanoparticles save their size.

This supports the previous assumption about

inability of DABB to modify Ag NPs into

agglomerates with hot spots. Obviously, the

saturation of central fragment, defining a

conjugation in the whole molecule, has a key

significance for linking ability of studied aromatic

amines. Revealing a role of central fragment

structure in the interaction of molecules with the

metal nanoparticles will be a subject of our future

investigation.

4 CONCLUSIONS

Based on the obtained results, we proposed the

models of adsorption for investigated stilbene dyes

(fig. 4). At sub-monolayer adsorption, bifunctional

aminostilbenes coordinate with the surface of two

Ag nanoparticles simultaneously that leads to the

formation of agglomerates with hot spots. The

stilbene derivatives with one amino group adsorb on

Ag nanoparticles by ordinary layer-by-layer way.

The bifunctional stilbenes with the tertiary amines

are also able to bind the metal nanoparticles, despite

the fact that tertiary amines are less active electron

density donors than primary ones.

Thanks to that, a list of photoactive compounds,

which can be used as promising modifiers of

plasmon substrates, can be significantly expanded.

However, the efficiency of bifunctional stilbenes

dyes as molecular linkers is lost in acidic solutions

and at the presence of chloride ions. At the same

time, the surface properties of single-particle

organometallic systems change due to the

redistribution of potential energy. The comparative

SERS study of bifunctional stilbenes and bibenzyls

showed that a conjugation in the molecule is also

significant for ability to link the nanoparticles.

The results of present study are able to find the

application points in the broad range of modern

research and development areas, connected to

photoactive materials. The revealed SERS response

of obtained organometallic composites dependent on

the molecular structure and concentration of

modifier as well on the environmental parameters

gives the deeper insight into surface chemistry and

practical possibilities of these and similar systems.

ACKNOWLEDGEMENTS

This work was supported by the Russian Science

Foundation (grant № 17-73-10209).

The experimental data was obtained using the

equipment of the Resources Centres of Saint-

Petersburg State University. The SERS spectra were

collected in the Center for Optical and Laser

Materials Research. The TEM images were obtained

in the Interdisciplinary Center for Nanotechnology.

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