Tuning of Chalcogenide Nanoparticles Fluorescence by Schiff bases

7/28/2019 Tuning of Chalcogenide Nanoparticles Fluorescence by Schiff bases

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Journal of Photochemistry and Photobiology A: Chemistry 254 (2013) 1219

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Journal ofPhotochemistry and Photobiology A:Chemistry

j ournal homepage: www.elsevier .com/ locate / jphotochem

Tuning ofchalcogenide nanoparticles fluorescence by Schiffbases

Selvaraj Naveenraj a, Sambandam Anandan a,, Sivan Velmathia, Abdullah M. Asiri b,Muthupandian Ashokkumarc

a Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, Indiab TheCenter of Excellence for AdvancedMaterials Research,King Abdul Aziz University, Jeddah21413, P.O. Box 80203, Saudi Arabiac School of Chemistry, University of Melbourne, Victoria 3010,Australia

a r t i c l e i n f o

Article history:Received 24 November 2012

Received in revised form 7 January 2013

Accepted 8 January 2013

Available online xxx

Keywords:

Fluorescence quenching, Chalcogenide

nanoparticles

Schiff bases

Electron transfer

Microwave chemistry

a b s t r a c t

The interaction between chalcogenide (CdS and CdSe) nanoparticles and Schiff bases in the presenceand absence of an electron withdrawing (nitro) substituent in organic media has been studied using

steady-state and time-resolved fluorescence measurements. The changes in the chalcogenide nanopar-

ticles luminescence properties in the presence of electron or hole acceptors provide information on

their electronic properties. For this purpose, platelet-like chalcogenide nanoparticles with average size

of512 nm were synthesized using a simple microwave technique and characterized using UVvis

spectroscopy, XRD and TEM. The fluorescence quenching studies suggest that Schiffbases quench the flu-

orescence ofchalcogenide nanoparticles effectively. Fluorescence lifetime studies suggest the presence

ofdynamic (collisional) encounters in the interaction ofschiffbases with the chalcogenide nanoparticles.

A possible quenching mechanism has also been proposed using Fourier transform infrared spectroscopy.

2013 Elsevier B.V. All rights reserved.

1. Introduction

Studies on chalcogenide nanoparticles have been at the focus of

intense research due to their unique size-dependent physicochem-

ical and optoelectronic properties [1]. This size quantization effect

allows chemists and material scientists, a distinct opportunity to

modify their surface properties in addition to their electronic and

chemical properties simply by controlling the particle size [2]. The

electronicproperties of nanoparticles can be studied by monitoring

their luminescence properties in the presence of electron or hole

acceptors [3,4].

Chalcogenide semiconductor nanoparticles, such as CdS and

CdSe, have been extensively studied due to their potential appli-

cations in various fields such as solar cells, field effect transistors,

light emitting diodes, photocatalysis, biological fluorescent labels

and biological sensors [57]. Among the various methods avail-able to synthesize chalcogenide nanoparticles [812], microwave

technique has attracted significant attention due to the dramatic

enhancement in reaction yields, reduction in reaction time, ease of

purification, use of less solvent and greater flexibility in reaction

conditions [1315].

Recently, many groups [3,4,1618] studied the interactions

of chalcogenide nanoparticles with different substrates using

Corresponding author. Tel.: +91 431 2503639; fax: +91 431 2500133.

E-mail addresses: [email protected], [email protected] (S. Anandan).

steady-state and dynamic fluorescencespectroscopy,which are the

main constructive tools in monitoring the electronic properties ofnanoparticles.

In this work, we have studied the interactions of chalco-

genide nanoparticles (CdS and CdSe) with Schiff bases in the

presence and absence of an electron withdrawing group. Schiff

bases, which are derived from the condensation of primary amines

and aldehydes or ketones and characterized by the anil linkage

HC N , possess structural similarities with natural biological

substances. They have a wide variety of applications in bio-

logical, inorganic, clinical and analytical fields [19,20]. They are

known to exhibit potent antimicrobial (antibactericidal, antiviral

and antifungal), anticonvulsant,anti-inflammatoryand insecticidal

activities [1921]. In addition some Schiff bases show pharma-

cologically useful activities like anticancer (radical scavenging

activity), anti-hypertensive, antifertility, analgesic, anthelmintic,and hypnotic activities [21].

2. Experimental

2.1. Materials

Cadmium chloride, sodium sulfide, sodium selenite, hydrazine,

salicylaldehyde, 4-nitroaniline and aniline were of analytical grade

purchased fromAldrichchemicalsand usedas received. Allsolvents

used were of extra pure analytical grade.

1010-6030/$ seefrontmatter 2013 Elsevier B.V. All rights reserved.

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S. Naveenraj et al. / Journal of Photochemistry andPhotobiologyA: Chemistry254 (2013) 1219 13

2.2. Methods

2.2.1. Microwave assisted synthesis of CdS nanoparticles

CdS nanoparticles were synthesized using the method by Yang

et al. [22] 0.05MNa2S, dissolved in distilled water (25ml), was

added drop-wise to a 100 ml round bottom flask containing 25ml

of 0.05M aqueous solution of CdCl2. The mixture was stirred using

magnetic stirrer and then ultrasonicated for homogeneous mixing.

After ultrasonication for 1 min, the round bottom flask was con-

nected with a reflux system and was kept in a domestic microwave

oven (2450MHz, Whirlpool). A water cooled condenser outside

the microwave oven cavity was connected by a glass joint to the

round bottomflask. The reaction was carried outunder microwave

irradiation for5 min. After cooling to ambient temperature,the yel-

low precipitate was centrifuged and washed and then dried under

vacuum at 60C for about 10h.

2.2.2. Microwave assisted synthesis of CdSe nanoparticles

0.05MNa2SeO3 dissolved in distilled water (25 ml) was added

drop-wise to the 100 ml round bottom flask containing 25ml of

0.05M aqueous solution of CdCl2. The mixture was stirred using

magnetic stirrer and 0.075M hydrazine was added to this mix-

ture while it was kept in an ultrasonic bath for reducing selenite

ion to selenide ion. After ultrasonication for 1 min, the round bot-

tom flask was connected with a refluxing system and was kept in a

domestic microwave oven (2450MHz, Whirlpool). A water cooled

condenser outside the microwave oven cavity was connected by a

glass joint to the round bottom flask. The reaction was carried out

under microwave irradiation for 5min. After cooling to ambient

temperature, the red precipitate was centrifuged and washed and

then dried under vacuum at 60C for about 10h.

2.2.3. Synthesis of Schiff bases N-salicylidene-4-nitroaniline

(SB1)& N-salicylideneaniline (SB2)

Schiff bases, N-salicylidene-4-nitroaniline & N-salicylidene-

aniline, were synthesized using a method by Sivakumar et al. [23]

salicylaldehyde (2 mmol in methanol) was slowly added to 4-nitro

aniline (2mM in methanol). The mixture was stirred for 3 h atroom temperature. The completion of the reaction was monitored

through TLC for the disappearance of the starting compounds. Then,

the solvent was evaporatedthroughrotary evaporator yielding red-

dish yellow crystals of N-salicylidene-4-nitroaniline. The yield was

about 98.3%. The solid thus obtained was dried in vacuum oven.

The melting point was found to be 130132 C. Following a same

procedure, N-salicylideneaniline was synthesized.

N-Salicylidene-4-Nitroaniline (SB1)

NN+

O

-O

HO

N-Salicylideneaniline (SB2)

N

HO

N-salicylidene-4-nitroaniline:

1H NMR(CDCl3, ppm), (aromatic) 6.9 m 2H, 7.2 m 1H, 7.4m 5H,7.6 d 1H (aromatic), 9.0 s 1H (CH = N), 12.3s, 1H (OH)

IR (KBr plates, (cm1): 1272, 1395, 1485, 1611, 3056, 3450.

UV max (nm): 355, 321.N-salicylideneaniline:1H NMR(CDCl3, ppm) 7.07.2 m 2H, 7.37.5 dd4H, 8.28.4 d 2H

(aromatic), 8.6 s, 1H (CH N), 12.5s, 1H (OH)

IR (KBr plates, (cm1): 1267, 1345, 1463, 1630, 3070, 3427.

UV max (nm): 336, 316, 300.

2.2.4. Interactions between microwave synthesized CdS and CdSe

nanoparticles and Schiff base N-salicylidene-4-nitroaniline

Microwave synthesized chalcogenide nanoparticles and Schiff

bases were soluble in dimethylformamide (DMF). 6105 M solu-

tion of Schiff base in DMF and 3105 M chalcogenide solution

were prepared. Twenty microliters of the prepared Schiff base

solution was added to 3ml of 3105 M CdS/CdSe solution. The

interactions were monitored by steady-state and time resolved

fluorescence spectroscopy.

2.3. Analytical procedures

Fourier transform infrared (FTIR) spectra were recorded usinga Perkin-Elmer 360 model IR double beam spectrophotometer.

The spectra were collected from 4000 to 400cm1 with 4 cm1

resolution over 40 scans. All spectra were collected against the

background spectrum of KBr. UVvis absorption spectra were

recorded on a T90+ UV/Visible Spectrophotometer (PG Instru-

ments, United Kingdom) in the range of 200800 at a scan

rate of 250nm/min. All spectra were collected against the back-

ground spectra of the solvents. Fluorescence emission spectra

were recorded on SHIMADZU Spectrofluorophotometer in the

range of 360600 nm at a scan rate of 500 nm/min. The CdS

and CdSe samples were excited at a wavelength of 340nm and

450nm, respectively. XRD spectra were recorded with a Philips

PW1710 diffractometer using Ni filtered Cu radiation. The sam-

ple was allowed to equilibrate with atmospheric moisture forat least 24h prior to recording. The scanning range was 2060

(2) with a step of 0.02 and a count time of 2 s. Fluorescencelifetime measurements were carried out in a picosecond time

correlated single photon counting (TCSPC) spectrometer with

tunable Ti-sapphire laser (TSUNAMI, Spectra physics, USA) as

the excitation source. In TCSPC experiment, the CdS and CdSe

samples were excited at a wavelength of 325 nm and 425 nm,

respectively. The fluorescence decay curves were analyzed using

the software provided by IBH (DAS-6). The surface morphol-

ogy and particle size of the synthesized nanoparticles were

analyzed using TEM (TECNAI G2 model). Samples were coated

on copper grid at normal atmospheric temperature and pres-

sure.

3. Results and discussion

3.1. Characterization of CdS and CdSe nanoparticles

Fig. 1a shows the absorption spectra of microwave synthe-

sized CdS and CdSe nanoparticles dissolved in DMF. They show

absorption onsets at 480 nm (2.58 eV) for CdS and 556 nm (2.23 eV)

for CdSe, whereas bulk CdS and CdSe particles show absorption

onsets at 512 nm (2.42 eV) and 716 nm (1.73 eV), respectively. The

apparent blue shift in the absorption onsets for both CdS and

CdSe can be attributed to the quantum-size confinement effect

[1,2]. From the absorption onset, the mean grain size of semi-

conductor nanoparticles can be determined using Brus equation


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14 S. Naveenraj et al. / Journal of Photochemistry andPhotobiologyA: Chemistry254 (2013) 1219

300 400 500 600 700 800

0.2

0.4

0.6

CdS

CdSe

Abs

orbance

Wavelength (nm)

400 500 600 700 8000100

200

300

400 CdSCdSe

Fluorescenceintensity

Wavelength (nm)

a

b

Fig. 1. Absorption spectra (a) and fluorescence spectra (b) of microwave assistedsynthesized cadmium sulfide and cadmium selenide nanoparticles.

[5,24].

Enanog = Ebulk

g +h2

8R2

1

me+

1

mh

1.8e2

40R

0.124e4

2(40)2

1

me+

1

mh

(1)

where Enanog is band gap of the sample

Enanog =hc

nano

, Ebulkg

is the bulk band gap, R is the radius of the particle, me is thereduced mass of electron (me = 1.73 10

31 kg), 0 is the vacuum

permittivity constant (0 = 8.851012 C2/N/m2) and is the

high-frequency dielectric constant of the semiconductor material(CdS =5.7; CdSe =7.3). From the above equation, the size of thenanoparticleR can be found out by rearranging the above equation

as:

R =

1.8e2/40

+

1.8e2/40

+ (Enanog E

bulkg )

h2

2

1/me + 1/m

h

2(Enanog E

bulkg )

(2)

The approximate sizes of CdS and CdSe nanoparticles were

found tobe 6nm and 10nm, respectively.

Fig. 1b shows the fluorescence spectra of microwave synthe-

sized CdS and CdSe nanoparticles in DMF excited at 340nm and

450nm, respectively. The emission spectrum for CdS nanoparticles

20 40 60 80

(311)(220)

CdSe nanoparticle

Inte

nsity

2 Theta ()

CdS nanoparticle

(111)

Fig. 2. X-ray diffraction patterns of microwave assisted synthesized cadmium sul-

fide and cadmium selenide nanoparticles.

is composed of three bands at 380nm (3.26 eV), 409nm (3.04eV)and 432 nm (2.87 eV) and that of CdSe nanoparticles is composed

of only one broad band at 596nm (2.08eV). It is reported that the

emission band occurs at 380nm for CdS nanoparticles with Cd:S

ratio of 1:1.2. However, with Ostwald ripening, surface-coated CdS

nanocrystals show an emission maximum at 409 nm for Cd:S ratio

of 1:2 [25,26].

The powder X-ray diffraction patterns of the as prepared sam-

ples are shown in Fig. 2. The XRD pattern of CdS nanoparticles

exhibits prominent,broad peaksat 2valuesof 27.0,43.6and 52.2,

which could be indexed to scattering from 111, 220, 311 planes,

respectively of cubic CdS. The XRD pattern of CdSe nanoparticles

exhibits prominent,broad peaksat 2valuesof 25.6,42.7and 49.7,which could be indexed to scattering from 11 1, 22 0, 3 11 planes,

respectively of cubic CdSe. These values are consistent with thedata reported in the literature [26,27]. The broadened peaks in the

XRD were used to calculate the average crystalline size using the

DebyeScherrer formula as 3.4nm and 5 nm, respectively [2427].

Fig. 3 shows TEM images of the microwave synthesized CdS and

CdSe nanoparticles. The TEM images show platelet-like particles

with average size of510nm (CdS) and 812 (CdSe). The regu-

lar behaviour of the lattice fringes inside the cluster suggests that

most clusters have good crystalline structure with no defects. The

overlapping ofclustersin someareas ofthe image is due tothe TEM

sample preparation [2426].

3.2. Fluorescence interaction studies of CdS and CdSe

nanoparticleswith Schiff bases

The fluorescence spectra of chalcogenide nanoparticles in the

absence and presence of Schiff bases are shown in Fig. 4. The fluo-

rescence intensity of chalcogenide nanoparticles was significantly

decreased without any change in the emission maximum and

spectral shape as the concentration of Schiff bases was increased,

this suggests that the Schiff bases quenched the fluorescence effec-

tively [16,17]. The fluorescence band of CdSnanoparticles (Fig.4a(i)

and (ii)) at 409 nm vanished when 1.2106 M N-salicylidene-4-

nitroaniline and 1.6106 M N-salicylideneaniline were added,

which may probably be due to a stronger interaction between

Schiff base and CdS nanoparticles. It can be inferred that nitro

substituted Schiff base actively interact with the chalcogenides

compared to Schiff bases without the nitro group [3]. The nor-

malized quenching intensities (F/F0; F and F0 is the fluorescence


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Fig. 3. Transmission electron micrographs of microwave assisted synthesized cadmium sulfide (a) and cadmium selenide (b) nanoparticles.

400 450 500 550 6000100

200

300

400

Flu

orescenceintensity

400 450 500 550 600

0

100

200

300

400

Fluorescenceintensity

500 600 700 8000100

200

300

400

Fluorescence

Intensity

Wavelength (nm) Wavelength (nm)

Wavelength (nm) Wavelength (nm)

500 600 700 800

0

100

200

300

400

FluorescenceIn

tensity

a(ii)a(i)

b(i) b(ii)

A A

FF

A

F

A

F

Fig.4. fluorescencespectraof (a)CdS nanoparticles and(b) CdSenanoparticles[4M] quenchedby Schiff bases (i) N-salicylidene-4-nitroaniline and(ii) N-salicylideneaniline

in theconcentration range of 02M. The concentrations of Schiff basesutilized in AF are0, 0.4, 0.8, 1.2, 1.6, and 2M.


5/8


5.0x104

1.0x105

1.5x105

2.0x105

2.5x105

0

1

2

3

4

CdS (381 nm) + SB1

CdS (381 nm) + SB2

F0

/(F

0-F)

1/[Q]

CdS (381 nm)+SB1(a)

(b)

CdS (433 nm)+SB1

CdS (381 nm)+SB2

CdS (433 nm)+SB2

CdSe +SB1

CdSe +SB2

0 5 10 15 2001

2

3

4

5

6

(F0

/F-1)

[Schiff base] x 10-7

Fig.5. SternVolmerplot(a) forCdS andCdSe nanoparticlesin thepresence ofSchiff

bases N-salicylidene-4-nitroaniline (SB1) and N-salicylideneaniline (SB2). Modified

SternVolmerplot (b) forCdS nanoparticles(381nm) in the presenceof Schiff bases

N-salicylidene-4-nitroaniline (SB1) and N-salicylideneaniline.

intensities of chalcogenide nanoparticles in the presence and

absence of the Schiff base, respectively) versus concentra-

tion of Schiff base suggests that N-salicylidene-4-nitroaniline

quenches chalcogenide nanoparticles more efficiently than that of

N-salicylideneaniline.

To analyze the dependence of the fluorescence intensity on

the Schiff base concentration, SternVolmer relationship has been

used.

F0F= 1+ KSV[Q] (3)

where, KSV is the SternVolmer constant which is a measure of

the efficiency of quenching and [Q] is the concentration of the

Schiff base [28]. By plotting (F0/F1) versus [Q], KSV can be cal-

culated from the slopes of the linear SternVolmer plots. In Fig. 5a,

the SternVolmer plots of chalcogenide nanoparticles (CdS and

CdSe) were shown. Within error limits, the SternVolmer plots

of CdS (except at 381nm) and CdSe are linear. From the slope of

these linear plots, we calculated the SternVolmer quenching con-

stant KSV and are tabulated in the Table 1. The result from the

fluorescence study indicated that the quenching effect of Schiff

bases on the fluorescence emission of chalcogenide nanoparti-

cles is found to be concentration dependent. The results from the

fluorescence study indicated that the quenching mentioned above

is not initiated by dynamic collision but from the ground state

complex formation as the quenching constants are greater than

that of scatter procedure (2.0102 Lmol1) [29]. The quenching

constant of N-salicylidene-4-nitroaniline (refer Table 1) is more

than that of N-salicylideneaniline irrespective of the chalcogenide

nanoparticles (CdS or CdSe), which indicates that the nitro group

of N-salicylidene-4-nitroaniline is actively involved in the quench-

ing process. The SternVolmer plots of CdS at 381 nm showed

upward curvature towardsYaxis which indicatesthat the presence

of both static and dynamic quenching [30]. Therefore the quench-

ing data was analyzed according to the modified SternVolmer

equation:

F0(F0 F)

=

1

faKa

1

[Q]

+

1

fa(4)

wherefa is thefractionof theinitial fluorescence andKa is theeffec-

tive quenching constant. A plot of (F0 F)/Fversus 1/[Q] (Fig. 5b)

gives a straight line where the values of Ka of CdS (381 n m)

has been found out to be 1.02105 and 5.89104 M1 for N-

salicylidene-4-nitroaniline and N-salicylideneaniline respectively.

This again supports the active involvement of the nitro group of

N-salicylidene-4-nitroaniline in the quenching process.

The fluorescence quenching can be dynamic or static or com-

bination of both. Dynamic quenching result from the collisional

encounters whereas static quenching results from the ground state

complex formation between the fluorophore and quencher. The

presence of dynamic quenching can be easily identified using life-

time measurements [31]. The fluorescence decay of chalcogenide

nanoparticles in the presence and absence of Schiff bases (Fig. 6)

clearly shows that the decay does not follow a single exponential

in all cases. Biexponential or higher terms are needed to fit the

experimental data. The multiexponential decay of the fluorescence

from chalcogenide nanoparticles is originated in surface traps gen-

eratedby thepresenceof excessCd2+ whichcorrespond to different

energy sublevels in which the electrons, excited by the absorp-tion of a photon, reside temporarily until they can interact with

trapped holes to produce radiative deactivation [9]. Average Fluo-

rescence decaytime of CdS and CdSe nanoparticles are shortened

when quencher concentration is increased, suggesting the pres-

ence of dynamic quenching i.e., photo-induced electron transfer

[32]. This decrease in decay time can be attributed to the depopu-

lation of the excited state as dynamic quenching is a rate process

[31].

The SternVolmer quenching plots of the time resolved fluo-

rescence quenching of chalcogenide nanoparticles (Fig. 7) were

obtained by plotting the ratio (0/) as a function of the quencherconcentration [Q]. The plots were found linear which indicates

that the quenching processes are dynamic in nature. The slope of

these linear plots gives dynamic quenching constant KD. The timeresolved fluorescence quenching constants of N-salicylidene-4-

nitroaniline (Table 1) is higher than that of N-salicylideneaniline

for both chalcogenide nanoparticles (CdS or CdSe), which follows

the same trend as that of the steady state fluorescence quenching.

This again confirms the active involvement of the nitro group in

the quenching process. The SternVolmer quenching constants

of the steady-state fluorescence quenching are greater than that

of the SternVolmer quenching constants of the time resolved

fluorescence quenching since SternVolmer quenching constants

of the time resolved fluorescence quenching contributed only by

the dynamic quenching whereas the SternVolmer quenching

constants of the steady-state fluorescence quenching contributed

by the combination of static and dynamic quenching occur for the

same fluorophore.


6/8


(c)

(a)

(b)

0 10 20 30 401

10

100

1000

Counts

Time (ns)

0 10 20 30 40 501

10

100

1000

10000

Counts

Time (ns)

0 10 20 30 40 501

10

100

1000

10000

Counts

Time (ns)

Fig. 6. Fluorescence decay curves of nanoparticles (4M) in the presence of Schiff

bases (i) N-salicylidene-4-nitroaniline (SB1) and (ii) N-salicylideneaniline (SB2) in

the concentrationof 0 (pink)and 2M(green).[(a)CdS+ SB1at375nm,(b)CdS+SB2

at 375nm, and (c) CdSe+ SB1 at 575nm].

The static quenching constant KS can be obtained, from the val-

ues ofKD as well as the intercept and the slope obtained in the plot

of [

F0 F/F

1/[Q]

] versus [Q], using the equation:

F0F

F

[Q]

= (KS + KD)+ KSKD [Q] (5)

Fig. 7. SternVolmer plot for time-resolved fluorescence quenching of CdS and

CdSe nanoparticles by Schiff bases N-salicylidene-4-nitroaniline (SB1) and N-

salicylideneaniline (SB2).

Static quenching constants are tabulated in Table 1. Predom-inantly the static quenching constant is greater than that of the

dynamic quenching constant which suggests that the quenching

mechanism is predominantly static than that of dynamic.

3.3. Mechanism for the fluorescence quenching

It was deduced that Schiff bases might interact with the chalco-

genide nanoparticles (CdS and CdSe) initiated through ground

state complex formationand then, dynamic i.e., collisional encoun-

ters. The FTIR spectrum provides the information of bond changes

that arises due to the interaction between chalcogenide nanopar-

ticles (CdS and CdSe) and Schiff bases. Fig. 8 shows the FTIR

4000 3000 2000 1000

SB2SB2 + CdS NPSB2 + CdSe NP

%T

ransmittance

Wavelength

4000 3500 3000 2500 2000 1500 1000 500

%

Transmittance

cm-1

SB1SB1 + CdSSB1 + CdSe

Around 1250 cm-1

Around 1250 cm-1(a)

(b)

Fig. 8. FT-IR spectra of N-salicylidene-4-nitroaniline (a) and N-salicylideneaniline

(b)in thepresence and absence of chalcogenide (CdS and CdSe) nanoparticles.


7/8


Table 1

Quenching constants obtained using steady-state and time-resolved fluorescence quenching.

Steady-state SternVolmer fluorescence

quenching constant KSV (Lmol1)

Time-resolved SternVolmer fluorescence

quenching constant or dynamic constant

KD (Lmol1)

Static constantKS (Lmol1)

N-salicylidene-4-

nitroaniline

(SB1)

N-

salicylideneaniline

(SB2)

N-salicylidene-4-

nitroaniline

(SB1)

N-

salicylideneaniline

(SB2)

N-salicylidene-4-

nitroaniline

(SB1)

N-

salicylideneaniline

(SB2)

CdS nanoparticle (381nm) 1.05105 3.18104 8.98105 1.78105

CdS nanoparticle (433nm) 7.35105 3.64105 1.98105 4.96104 5.51105 2.48105

CdSe nanoparticle 7.017105 4.65105 3.67105 2.18104 1.72105 3.73105

Fig. 9. Mechanism for fluorescence quenching of chalcogenide nanoparticles by

Schiff bases.

spectra of Schiff bases in the absence and presence of chalco-

genide nanoparticles. In both cases, there is a slight shift in the

peak position around 1250cm1 with increase in the peak inten-

sity which corresponds to the azomethine group of Schiff base.

This indicates that the quenching of fluorescence from chalco-

genide nanoparticles arises due to the efficient charge-transfer

fromelectron-hole separation process of nanoparticles to the nitro-

gen atom belonging to the azomethine group of Schiff base. But in

the case of N-salicylidene-4-nitroaniline, in addition to the peak

shift around 1250cm

1

, the C N stretching peak position shiftfrom 1435cm1 (free N-salicylidene-4-nitroaniline) to 1418cm1

(CdS. N-salicylidene-4-nitroaniline complex) after the addition of

CdS nanoparticles. It follows the same trend in the case of CdSe

nanoparticles. These results also suggest that the quenching of flu-

orescence from chalcogenide nanoparticles is due to the efficient

charge-transfer from electron-hole separation of nanoparticles to

thenitrogen atom belongingto theazomethine group of Schiff base

and the Nitro group of N-salicylidene-4-nitroaniline. This mecha-

nism is illustrated in Fig. 9.

4. Conclusions

Platelet-like CdS and CdSe nanoparticles with average sizeof 510 nm and 812 n m were synthesized using a sim-

ple microwave technique. The interactions among chalcogenide

nanoparticles and Schiff bases with and without electron with-

drawing group were investigated using fluorescence studies. The

results showed that Schiff base with a nitro substituent (N-

salicylidene-4-nitroaniline) is an efficient quencher. Schiff bases

might interact with the chalcogenide nanoparticles initiated

through ground state formation and then through collisional

encounters (dynamic). Lifetime measurements suggested the pres-

ence of the dynamic quenching processes and the interaction has

been further discussed using FT-IR spectroscopy. The incredible

high quenching efficiency of Schiff bases will provide a way to con-

trol the luminescence properties of the chalcogenide nanoparticles

with a tiny quantity of the additive.

Acknowledgements

Author SA and MA thanks DST, New Delhi (INT/AUS/P-1/07

dated 19th Sep 2007) and DEST, Australia for the sanction of

INDIA-AUSTRALIAN strategic research fund for their collaborative

research. Author SA thank DST, New Delhi (SR/S1/PC-49/2009) for

major research project. Authors SA and SV thank DST for sanc-

tioning FIST (SR/FST/CSI-190/2008 dated 16th March 2009) and

Nanomission (SR/NM/NS-27/2008, dated 25th Feb 2009) projects.

Alsothe authors thankProf. P. Ramamurthy, Director, National Cen-

tre for Ultra Fast Processes, University of Madras for time-resolved

fluorescence measurements.

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