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Sensors and Actuators B 209 (2015) 15–24

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

highly selective colorimetric and turn-on fluorescent chemosensorased on 1-(2-pyridylazo)-2-naphthol for the detection ofluminium(III) ions

inod Kumar Guptaa,b,∗, Sudhir Kumar Shooraa, Lokesh Kumar Kumawata,jay Kumar Jaina

Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, IndiaKing Fahd University of Petroleum and Minerals Dhahran, Saudi, Saudi Arabia

r t i c l e i n f o

rticle history:eceived 10 September 2014eceived in revised form 31 October 2014ccepted 31 October 2014

a b s t r a c t

A new azo compound, 1-(2-pyridylazo)-2-naphthol, has been prepared and characterized. The synthe-sized compound shows a very high affinity for aluminium. The complexation reaction of aluminiumhas been explored by Colorimetry, FT-IR, 1H NMR, HRMS and fluorescence spectroscopy. The detectionlimit has been found to be 1.81 × 10−8 M estimated by the titration method. The turn-on fluorescence

vailable online 20 November 2014

eywords:luorescenceolorimetryluminium(III) ions

behaviour of aluminium interaction with ligand has been found to be so fast (<5 s) that it can be used forits qualitative as well as quantitative estimation.

© 2014 Elsevier B.V. All rights reserved.

-(2-pyridylazo)-2-naphthol

. Introduction

The occurrence of aluminium in earth crust is widespread1,2]. As a result of most abundant availability, aluminium metals available cheaply and finds extensive use in home construc-ion, transport, household appliances and machinery. It is a goodonductor of electricity and therefore used for the manufacturef electric wires, and various electrical and electronic appliances3–5]. The excessive use of aluminium has resulted in its pres-nce in various environmental hazards and food chain. Hence its slowly consumed by human beings where it causes many toxicffects, such as neurotoxicity, Parkinson’s and Alzheimer’s diseasesnd cancer [6–12]. It also affects iron metabolism which may causeernicious anaemic condition. In view of widespread applicationsf aluminium in domestic life and its toxic effects [13], its determi-ation in environment is vastly important.

A number of sophisticated instrumental analytical techniquesay be used such as voltammetry [14–18], atomic absorp-

ion spectroscopy (AAS) [19,20], inductively coupled plasma

∗ Corresponding author at: Department of Chemistry, Indian Institute of Technol-gy Roorkee, Roorkee 247667, Uttarakhand, India. Tel.: +91 1332285801;ax: +91 1332273560.

E-mail addresses: vinodfcy@iitr.ac.in, vinodfcy@gmail.com (V.K. Gupta).

ttp://dx.doi.org/10.1016/j.snb.2014.10.143925-4005/© 2014 Elsevier B.V. All rights reserved.

emission spectrometry (ICP-AES) [21], and potentiometric [22–33]and spectrophotometric sensors [34–45]. Of these techniques, thedetermination of aluminium by sensors has been found to be expe-dient, fast and can be used for the analysis of large number ofsamples in a very short period of time. The problem is, that in spiteof many merits of sensors, its use is limited due to poor selectiv-ity and sensitivity. Thus a more selective and sensitive sensor isrequired to be developed.

During our synthetic studies of azo compounds, we have foundthat an azo compound 1-(2-pyridylazo)-2-naphthol (PAN) syn-thesized by us interacts strongly and selectively with aluminiumions and has therefore been used for preparing colorimetric andfluorometric sensor for aluminium ions. The results of these inves-tigations are reported in the present communication.

2. Experimental

2.1. Reagents, materials and apparatus

2-aminopyridine, �-naphthol and sodium hydroxide of analyt-

ical grade were procured for synthesis of PAN from Merck (India)and further used without any purification. Sodium nitrite was pro-cured from Samir tech-chem industry (India). Chloride and nitratemetal salts are purchased from SD-Fine chem. Limited.

16 V.K. Gupta et al. / Sensors and Actuators B 209 (2015) 15–24

d PAN

ECs(MUsR5pfm

Fig. 1. (a and b) FT-IR spectra of PAN an

The IR spectra of chemosensor were recorded on a Perkinlmer FT-IR spectrometer in the range 4000–400 cm−1. TheHNS data were collected on a vario MICRO cube and 1H NMRpectra of chemosensor were recorded on a Bruker 500 MHzUSA), using TMS as an internal standard, CD3OD as solvent.

ass spectra were recorded on Bruker-MicrOTOF II (USA). TheV–vis absorption spectra were recorded on a Shimadzu UV-2450

pectrophotometer and the Fluorescent spectra on a ShimadzuF-5301PC spectrofluorophotometer with slit width value of

.0 and 3.0 nm. All the metal solutions for the study wererepared in methanol solution. Stock solution of PAN and dif-erent metal chlorides and nitrates (10 mM) were prepared in

ethanol.

-Al complex (in methanolic solution).

2.2. Synthesis of ligand PAN

The ligand, PAN (Scheme 1), was prepared in aqueous mediaby the reported method with a slight modification [46]. 5.0 mmolof 2-aminopyridine was dissolved in 100 ml of water with con-centrated HCl (5.0 ml). The resulting solution was cooled to 0–5 ◦Cwith ice and an aqueous cold solution of sodium nitrite (24.0 mmol)was added under stirring. After about 2 min, a cold solution of �-naphthol (2.0 mmol) in aqueous sodium hydroxide (8.0 ml; 0.2 M)

was added. The formation of a coloured azo compound wasobserved. This mixture was stirred for about 10 min and then fil-tered and air-dried. Colour: brick red; yield: 0.350 g (70%); state:solid; m.p.: 139–141 ◦C.

V.K. Gupta et al. / Sensors and Act

N

NH2

HO

NaOH

N NN

HO

+

1-(2-pyridylazo)-2-naphthol

3

3

c1

no111TntCpH7(

Scheme 1. Synthesis of PAN.

. Results and discussion

.1. Characterization of PAN and its complex with aluminium

The FT-IR spectrum of PAN (Fig. 1a) shows prominent peaks (�,m−1) at: 3324 (methanolic O H), 2943, 2831 (aliphatic C H),621 ( C N), 1448, 1413 (aromatic C C ), 1271, 1201 (phenolicO H), 1113, 1022 (aliphatic C O), 736 (ortho substituted phe-ol). A comparison of IR spectra (Fig. 1a) of the ligand and spectraf the aluminium complex (Fig. 1b) shows that the ligand peaks at621 cm−1 due to C N group are shifted at 1640 cm−1; 1271 and201 cm−1 due to O H group are merged into one and appear at323 cm−1 indicating that C N and O H are coordinating sites.hus FT-IR spectra support that Al forms, complex with PAN withitrogen and oxygen, act as donor sites. The elemental analysis ofhe PAN was done and the results are: Anal. Calc. % for C15H11N3O:

72.20; H 4.41; N 16.85. Found %: C 71.52; H 4.4; N 16.71. The

rominent 1H NMR peaks (Fig. 2) were found at (CD3OD) (ı, ppm (J,z)): 6.60 (d, J = 9.5, 1H), 7.20–7.22 (m, 1H), 7.41 (dt, J = 1.0, 7.5, 1H),.50–7.55 (m, 2H), 7.75 (d, J = 10, 1H), 7.91–7.93 (m, 2H), 8.34–8.37m, 2H) and also support the structure of PAN as shown in Scheme 1.

Fig. 2. 1H NMR spectra of the 1-(2-p

uators B 209 (2015) 15–24 17

Further, HRMS spectra (Fig. 3) showing a prominent peak at272.0780 due to L+Na+ also supports the alleged structure of PANas given in Scheme 1. The spectra of aluminium complex with PANalso give a prominent peak at 277.9203 due to L+Al+H+ as shownin Fig. 4. Hence the HRMS spectra indicate that the stoichiometryof ligand and Al-complex is 1:1.

3.2. UV–vis absorption spectral studies

The UV–vis absorption spectrum of the metal and ligand mix-ture recorded in methanolic solution (50 �M) is shown in Fig. 5which reveals absorption band at 464 nm. Different metal ionsformed colour complexes with the ligand that are exhibited inFig. 6; aluminium ions formed a dark red colour complex, whereasthe other metal complexes were found comparatively lighter. It isseen clearly that most of the metal ions (Zn2+, Cd2+, Cu2+, Ni2+, Gd3+,Mn2+, Nd3+, Co2+, Fe2+, Fe3+, Pb2+ and Al3+) form complexes withPAN in the absorption band region of 490–650 nm. Whereas othermetal ions like Ba2+, Ca2+, Cr3+, Cs+, Hg2+, K+, Li+, Mg2+, Na+ and Sr2+

do not react significantly with the ligand.

3.3. Fluorescence emission studies

The reported ligand (10 �M) shows a weak fluorescence emis-sion band at 521 nm with an excitation of 490 nm whereas whenother metal ions (10 �M) such as Ba2+, Ca2+, Co2+, Cd2+, Cr3+, Cs+,Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Na+, Mg2+, Mn2+, Nd3+, Pb2+, Sr2+, Gd3+,

Zn2+ and Ni2+ were added to the ligand, then there were no signif-icant changes observed in fluorescence emission spectra of metalligand complex. Upon addition of aluminium significant fluores-cence enhancement accompanied by a red shift of 48 nm from 521

yridylazo)-2-naphthol (PAN).

18 V.K. Gupta et al. / Sensors and Actuators B 209 (2015) 15–24

-(2-py

tiiUpwb

iic

Fig. 3. HRMS spectra of the 1

o 569 nm was noticed (Fig. 7). The enhancement in fluorescencentensity is observed that exhibits “on–off” mode of high sensitiv-ty towards aluminium(III) ions. In addition to this, the receptor onV light treatment shows remarkable changes from colourless toinkish red fluorescence in the presence of aluminium within 5 s,hich can be easily detected by the naked eye. Thus this ligand can

e used to detect aluminium visually.

Furthermore, the fluorescence response of the ligand for var-

ous concentrations of aluminium(III) ions (0–50 �M) was alsonvestigated. Upon addition of aluminium(III) ions, the fluores-ence intensity centred at 569 nm of receptor gradually increased

ridylazo)-2-naphthol (PAN).

and remained approximately steady when 1.0 eq. of Al3+ ions wasadded, indicating the formation of a 1:1 bonding mode betweenligand and aluminium(III) ions (Fig. 8). In addition, a Job plotobtained from the emission data showed the 1:1 stoichiometriccomplexation (Fig. 9).

The selectivity of the ligand (10 �M) for Al3+ ions over othermetal ions was also investigated (Fig. 10). It is seen that the fluo-

rescence emission intensity of aluminium ligand mixture remainsunaffected by Ba2+, Ca2+, Cr3+, Cs+, Fe3+, Gd3+, Hg2+, K+, Li+, Na+,Mg2+, Mn2+, Nd3+, Pb2+ and Sr2+. Therefore, aluminium has selec-tivity over these metals and the synthesized receptor can be used

V.K. Gupta et al. / Sensors and Actuators B 209 (2015) 15–24 19

a of th

fhfa

Fig. 4. HRMS spectr

or its estimation in the presence of these metals. On the otherand metal ions such as Co2+, Cd2+, Cu2+, Fe2+, Zn2+ and Ni2+ were

ound to decline emission intensity of aluminium-PAN complexnd therefore expected to cause interference. This interference

e PAN-Al complex.

effect could be reduced by addition of higher concentration of alu-minium(III) ions.

Fig. 11 depicts the changes in fluorescence intensity of the recep-tor before and after the addition of aluminium(III) ions in the

20 V.K. Gupta et al. / Sensors and Actuators B 209 (2015) 15–24

300 40 0 50 0 60 0 70 0

0.00

0.25

0.50

0.75

1.00

1.25

Pb2+

Gd3+

Nd3+Mn2+

Al3+

Fe3+

Fe2+

Co2+

Cu2+

Ni2+

Cd2+

Zn2+

PAN, Ba2+, Ca2+, Cr3+,Cs+, Hg2+, K+, Li+, Mg2+, Na+, Sr2+

Abs

orba

nce

Waveleng th (nm)

Fig. 5. Absorption spectra of (50 �M) methanolic solution of ligand in the presence of different metals (Ba2+, Ca2+, Co2+, Cd2+, Cr3+, Cs+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Na+,Mg2+, Mn2+, Nd3+, Pb2+, Sr2+, Gd3+, Zn2+, Ni2+ and Al3+) (50 �M) in methanolic solution.

Fig. 6. Colour changes of (50 �M) concentration of ligand with different metal ions (50 �M) in methanol in 1:1 (v/v, mL) ratio.

500 55 0 60 0 65 0 70 0

0

200

400

600

800

PAN, Ba2+, Ca2+, Co2+, Cd2+, Cr3+, Cs+, Cu2+, Fe2+, Fe3+, Hg2+,K+, Li+, Mg2+, Mn2+, Na+, Nd3+,Ni2+, Pb2+, Sr2+, Zn2+, Gd3+

Al3+

Fluo

rese

nce

Inte

nsity

(au)

Waveleng th (nm)

Fig. 7. Fluorescence emission spectra of ligand (10 �M) in the presence of different metal ions (Ba2+, Ca2+, Co2+, Cd2+, Cr3+, Cs+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Na+, Mg2+, Mn2+,Nd3+, Pb2+, Sr2+, Gd3+, Zn2+, Ni2+ and Al3+) (10 �M) in methanol solvent using slit width of 5 nm.

V.K. Gupta et al. / Sensors and Actuators B 209 (2015) 15–24 21

500 55 0 60 0 65 0 70 00

50100150200250300350400450500550600650700750800

0 1 2 3 4 5-50050100150200250300350400

Abs

orba

nce

0.0 M

50 M

Fluo

rese

nce

Inte

nsity

(au)

Waveleng th (nm)

Fig. 8. Effect of the increasing aluminium ion concentration (0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 10, 25 and 50 �M) on emission spectra of the ligand (10 �M) witha n of t

piqd

eE

n excitation of 490 nm. Inset shows the fluorescence change at 569 nm as a functio

resence of other sensed metal ions under UV lamp. It has exhib-ted that the other metal ions (Cd2+, Co2+, Cu2+, Fe2+, Ni2+ and Zn2+)uenches the fluorescence intensity of the receptor-Al3+ complex,ue to the similar binding ability towards these metal ions.

On the other hand, the reversibility of the ligand is consid-red as an important aspect in practical applications, thereforeDTA titration was conducted to scrutinize the reversibility of

0.0 0.2 0.4 -50

0

50

100

150

200

250

300

350

400

Fluo

resc

ence

Inte

nsity

(au)

[Host]/ ([H

Fig. 9. Job’s plot for receptor by fluorescence method (�em = 569 nm) using sl

he amount of Al3+ ions at a slit width of 3.0 nm.

the aluminium-PAN complexation in Fig. 12. Results have shownthat aluminium-PAN complex emission intensity declined after theaddition of EDTA solution at 569 nm, because EDTA in the solutionreacts first with the available free metal ions in the solution, and

then it displaces the metal ions from the ligand-metal ion complex.Due to this effect the fluorescent of metal ligand complex goes to“turn-off”.

0.6 0.8 1.0

ost] +[Al3+])

it width of 3.0 nm; total concentration of receptor and metal is 10 �M.

22 V.K. Gupta et al. / Sensors and Act

Fig. 10. Selectivity of the receptor towards Al3+ and other metal ions. In the absence(red bars) and presence (green bars) of 1.0 eq. Al3+ ion at �ex = 490 nm, slit widthwas taken at 3.0 nm during the experiment at room temperature in methanol. (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

Fig. 11. The fluorescence emission changes of sensor (10 �M) with 1.0 eq. of Al3+, and icommercially available UV lamp (�ex = 490 nm).

Fig. 12. The variation in the fluorescence intensity on the increasing concentration of E(10 �M) with an excitation of 490 nm at slit width of 3.0 nm. Inset is the plot between flu

uators B 209 (2015) 15–24

3.4. 1H NMR titration

The 1H NMR spectra of PAN with and without addition of alu-minium(III) ions are shown in Fig. 13. The results showed thatwhen metal-ligand titrations were performed by the addition ofdifferent equivalents of aluminium(III) ions, some changes in the1H NMR-spectra were observed. It has been found that the peakof protons at about 8.05 and 7.95 ppm of ligand were shifted toupfield 8.0 and 7.90 ppm, followed by the addition of aluminium(III)ions, due to interruption of the intra-molecular hydrogen bond-ing between the phenolic hydroxyl group and the nitrogen of theazo moiety. Conversely, the protons of pyridine ring were shifteddownfield with the addition of aluminium(III) ions, which indicatedthat the structure of receptors became more rigid after coordina-tion with aluminium ions. This indicates that the phenolic hydroxylgroup, nitrogen atom of the azo moiety and nitrogen of pyridinering participated in complexation with aluminium ions. There-fore, it can be supposed that the aluminium ion may be chelated

by the counter anion or solvent in order to satisfy the need ofsix-coordination.

n the presence of other metal ions (Cd2+, Co2+, Cu2+, Fe2+, Ni2+, Zn2+) excited by a

DTA (0.0, 5, 10, 15, 20, 25, 30, 35, 40, 45 and 50 �M) in the presence of the ligandorescence intensity vs EDTA equivalent.

V.K. Gupta et al. / Sensors and Actuators B 209 (2015) 15–24 23

1 f recep 3+

4

issccuoa

A

UR

R

[

[

[

[

[

[

[

[

[

[

[

[

[

Fig. 13. H NMR (500 MHz) spectra o

. Conclusion

The ligand prepared can be used to detect aluminium ions qual-tatively and quantitatively both by colorimetry and fluorescencepectrophotometry. However, fluorescence method shows higherelectivity and it can be used to estimate aluminium(III) ions con-entration with a detection limit of 1.81 × 10−8 M. The receptoran also be used to detect aluminium(III) ions in various samplesnder the UV light even through the naked eye. Consequently it isf great utility for analysing large number of biological, analyticalnd environmental samples.

cknowledgement

One of the authors (Sudhir Kumar Shoora) is thankful to theniversity Grants Commission/Council of Scientific and Industrialesearch, New Delhi for the award of Senior Research Fellowship.

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Biographies

Vinod Kumar Gupta obtained his Ph.D. degree in chem-istry from the University of Roorkee (now Indian Instituteof Technology Roorkee) Roorkee, India, in 1979. Since thenhe is pursuing research at the same Institute and presentlyholding the position of Professor, Chemistry Department,at Indian Institute of technology Roorkee, Roorkee. Heworked as a post-doctoral fellow at University of Regens-burg, Germany, in 1993 as an EC fellow and was DAADvisiting professor at University of Chemnitz and Freie Uni-versity of Berlin in 2002. He has published more than400 research papers, many reviews and two books whichfetched him more than 27,000 citations with h-index of110. He was awarded the Indian Citaton Laureate Award

in 2004. His research interests include chemical sensors, waste water treatment,environmental and electro analytical chemistry. Dr. Gupta is an elected Fellow ofthe World Innovation Foundation (FIWF) since July 2004 and Fellow of the NationalAcademy of Sciences (FNASc) since 2008.

Sudhir Kumar Shoora has joined IIT-Roorkee as aResearch Scholar (JRF) in 2011 after completed his M. Phil.in 2010 from M.G.S. University, Bikaner. He had completedhis Master of Science in Applied Chemistry from M.D.S.University, Ajmer in 2009. Currently, he is funded by Uni-versity Grant Commission (UGC), New Delhi (India).

Lokesh Kumar Kumawat is a Ph.D. student and pursuingresearch on chemical sensor under the supervision of V.K.Gupta and A.K. Singh at IIT Roorkee. He has completedhis Master degree in Chemistry from MLSU (Mohan LalSukhadia University) Udaipur, India in 2009.

Professor A.K. Jain was working as an emeritus professor at Indian Institute ofTechnology, Roorkee. After serving as a regular professor in the same institute for20 years, he has 150 publications and has supervised Ph.D. programmes of about 40students.