Tuning FRET efficiency as a novel approach for improved detection of naphthalene: application to environmental samples

Article abstract of DOI:10.1002/jmr.2531

Naphthalene has emission in the ultraviolet (UV) region, limiting its trace level determination in biological and environmental samples due to detrimental effect of UV light on the living cell and interference from other substances having emission in the

Full text of DOI:10.1002/jmr.2531

Research article
Received: 11 June 2015,
Revised: 25 November 2015,
Accepted: 4 December 2015,
Published online in Wiley Online Library: 21 January 2016
(wileyonlinelibrary.com) DOI: 10.1002/jmr.2531
Tuning FRET efficiency as a novel approach
for improved detection of naphthalene:
application to environmental samples
Sandip Nandia, Sangita Adhikaria, Sandip Mandala, Arnab Banerjeeb**
and Debasis Dasa*
Naphthalene has emission in the ultraviolet (UV) region, limiting its trace level determination in biological and
environmental samples due to detrimental effect of UV light on the living cell and interference from other substances
having emission in the UV region. Fluorescence resonance energy transfer strategy is adopted for determination of
traces naphthalene in the visible region. Significant improvement of lowest detection limit of naphthalene has been
achieved through tuning of fluorescence resonance energy transfer efficiency. Anthranilic acid pyrene (ANP) conjugate
provides lowest detection limit for naphthalene among three probes studied, viz. ANB, aniline- pyrene conjugate
(APA) and ANP. ANP efficiently measures naphthalene content in river water. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: FRET; naphthalene; anthranilic acid; pyrene
aim is to develop a strategy that shifts naphthalene emission
to visible region. Several photo sensing processes, viz. photo-
INTRODUCTION
Polycyclic aromatic hydrocarbons (PAHs) fall within the 10 most
toxic classes of compounds, categorized by Centre for Disease
Control in 2011 (Serio et al., 2013). Naphthalene, the smallest
member of PAH family, widely used as moth repellent for clothes
or blankets sublimes easily into the atmosphere. Hence, contam-
ination of naphthalene in environment, viz. air, food and water,
has become a serious problem. It damages red blood cells and
causes nausea, vomiting, diarrhoea and blood excretion in urine.
It also causes cataracts causing cloudy vision to animals contam-
inated by naphthalene via food and breath (Agency for Toxic
Substances and Disease Registry, ATSDR, 1996; U.S. Environmen-
tal Protection Agency, 1994). Aqueous solubility is a fundamental
parameter in evaluating the toxicity of PAHs as it allows spread-
ing over the living systems making contamination easier.
Because of higher water solubility of naphthalene over other
PAHs, its trace level detection and estimation is important.
Methods for naphthalene determination are mainly based on
high-performance liquid chromatography (Diaz et al., 1999;
Kishikawa et al., 2003; Gautam et al., 2011) or other chroma-
tography techniques (Hossain et al., 2014; Andreoli et al., 1999).
Fluorescence method has several advantages over chromato-
graphic detection such as easy sample preparation protocols,
convenient operation, fast response, low cost and high sensitivity
(Zhu et al., 2010; Han et al., 2012; Anbu et al., 2012).To the best of
our knowledge, fluorescence probe for selective trace level
determination of naphthalene is not reported yet. Presently, our
research focuses to develop fluorescence probes selective for
cations (Banerjee et al., 2012; Sahana et al., 2011; Das et al.,
2013) anions (Banerjee et al., 2013a; Sahana et al., 2012; Lohar
et al., 2013), amino acids (Das et al., 2011; Banerjee et al., 2013b)
and small molecules. Naphthalene has emission at ultraviolet
region, making difficult for its fluorescence determination. Our
induced electron transfer (Kim and Quang, 2007; Aoki et al.,
1992; Ji et al., 1999; Leray et al., 1999; Leray et al., 2001), intermolec-
ular charge transfer (Xu et al., 2005; Wang et al., 2006), chelation-
enhanced fluorescence (CHEF) (Lim et al., 2005; Guha et al., 2012;
Das et al., 2012), excimer/exciplex formation (Weller, 1956) and
fluorescence resonance energy transfer (FRET) (Förster, 1948;
Förster, 1949; van der Meer et al. 1994), are available for selective
determination of the analytes. Herein, we have synthesized
three new fluorescence probes (ANP, APA and ANB) based on
pyrene/anthranilic acid as FRET acceptor whereas naphthalene
functions as donor. All three probes are able to shift naphthalene
emission into visible region. Anthranilic acid, chosen for water
solubility, in conjugation with another aromatic ring, behaves as
a good FRET acceptor. Pyrene is chosen for its π-stacking ability
with naphthalene. The lowest detection of limit (LOD) for naphtha-
lene is significantly improved by tuning the FRET efficiency via
changing the acceptor to pyrene-anthranilic acid conjugate.
Finally, Gomti river water (India) is analysed to determine the
naphthalene content (Malik et al. 2011).
* Correspondence to: D. Das, Department of Chemistry, The University of
Burdwan, Golapbag, Burdwan, India.
E-mail: ddas100in@yahoo.com
** A. Banerjee, Department of Chemistry & Biochemistry, The University of Texas
at Austin, Austin, TX, USA.
E-mail: ab.bur@utexas.edu
a S. Nandi, S. Adhikari, S. Mandal, D. Das
Department of Chemistry, The University of Burdwan, Golapbag, Burdwan,
India
b A. Banerjee
Department of Chemistry & Biochemistry, The University of Texas at Austin,
AustinTX, USA
J. Mol. Recognit. 2016; 29: 303–307
Copyright © 2016 John Wiley & Sons, Ltd.

S. NANDI ET AL.
Synthesis of N-phenyl-1-(pyren-1-yl) methanamine (APA)
(Scheme 1)
EXPERIMENTAL
Materials and methods
To the methanol solution of aniline (0.034 g, 0.365 mmol),
pyrene-1-carboxaldehyde (0.084 g, 0.365 mmol) was dissolved
and refluxed under atmospheric pressure at 80 °C for 4 h. A dark
yellow coloured reaction mixture was obtained and cooled un-
der room temperature, and a solid product was obtained after
2 days. The residue was recrystallized from methanol Yield, 80
%. The product was characterized by: 1H NMR (Figure S4)
(400 MHz, DMSO-d6) δ (ppm): 7.33 (1H, t, J = 8.8), 7.51 (4H, m),
8.17 (1H, d, J = 7.6), 8.25 (1H, d, J = 9.2 ), 8.31 (1H, d, J = 8.8),
8.39 (1H, d, J = 8.0), 8.48 (1H, d, J = 8.4), 8.60 (1H, m), 8.78 (1H,
d, J = 8.4), 9.29 (1H, d), 9.42 (1H, d), 9.64 (1H, s). QTOF-MS ES+
(Figure S5), m/z (M + H)+, calculated for C23H16N: 306.3875,
Pyrene-1-carboxaldehyde, anthranilic acid, benzaldehyde and
aniline have been purchased from Sigma Aldrich (India). Spectro-
scopic grade solvents have been used. Other chemicals are of
analytical reagent grade and used without further purification.
Mili-Q 18.2 MΩ cmÀ1 water has been used throughout all the
experiments. A Shimadzu Multi Spec 1501 spectrophotometer
is used for recording ultraviolet–Vis spectra. Fourier transform
infrared (FTIR) spectra are recorded on a PerkinElmer FTIR (model
RX1, USA) spectrophotometer. Mass spectra are obtained using
QTOF Micro YA 263 mass spectrometer in ES positive mode.
Proton nuclear magnetic resonance (1H NMR) spectra have been
recorded using Bruker Avance 400 (400 MHz) instrument in
dimethyl sulfoxide (DMSO)-d6. Time-resolved fluorescence life-time
measurements were performed using a picosecond pulsed diode
laser-based time-correlated single photon counting spectrometer
from IBH (London, UK) at λex = 280 nm and Microchannel Plate
Photomultiplier Tubes (MCP-PMT) as a detector. The emission from
the sample was collected at a right angle to the direction of the ex-
citation beam maintaining magic angle polarization (54.71). The full
width at half maximum of the instrument response function was
250 ps, and the resolution was 28 ps per channel. The data were
fitted to multiexponential functions after deconvolution of the
instrument response function by an iterative reconvolution tech-
nique using IBH DAS 6.2 data analysis software in which reduced
w2 and weighted residuals serve as parameters for goodness of fit.
found: 306.3. FTIR (neat) (Figure S6) νmax = 1577 cmÀ1
.
Synthesis of 2-(benzylideneamino) benzoic acid (ANB)
(Scheme 1)
ANB has been prepared with 70% yield by conjugating
anthranilic acid (0.05 g, 0.365 mmol) with benzaldehyde
(0.039 g, 0.365 mmol) in methanol. A dark brown product
obtained was recrystallized from methanol. The product was
characterized by: 1H NMR (Figure S7) (400 MHz, DMSO-d6) δ
(ppm): 6.51 (2H, m), 6.73 (2H, m), 7.23 (2H, d, J = 8.4), 7.60 (1H,
d, J = 7.6), 7.78 (2H, d, J = 6.8), 7.95 (1H, t, J = 6.8). QTOF-MS ES+
(Figure S8), m/z (M + H)+, calculated for C14H12NO2: 226.0863,
found: 226.1155. FTIR (neat) (Figure S9) νmax = 1586, 1611 and
1650 cmÀ1
.
Synthesis of 2-((pyren-1-ylmethylene) amino) benzoic acid
(ANP) (Scheme 1)
Results and discussion
Among three probes, two have been prepared by condensation
of pyrene-1-carboxaldehyde with anthranilic acid (ANP) and
aniline (APA), while the third one involves condensation of benz-
aldehyde with anthranilic acid (ANB) (Scheme 1). The probes are
A 0.050 g Anthranilic acid (0.365 mmol) was dissolved in
methanol. To this solution, pyrene-1-carboxaldehyde (0.084 g,
0.365 mmol) was added and refluxed under atmospheric pres-
sure at 80 °C for 4 h. The red coloured reaction mixture was
cooled under room temperature, and the clear solution was kept
for 2 days. The solid product obtained was recrystallized from
1
characterized by H NMR, QTOF-mass and FTIR spectra (Figures
S1–S9, Electronic Supplementary Information (ESI)). ANP, APA
and ANB have absorbance at 360, 365 and 335 nm, respectively
(Figure 1), allowing as FRET acceptor for naphthalene (λem
1
methanol with 75% yield. The product was characterized by: H
,
NMR (Figure S1) (400 MHz, DMSO-d6) δ (ppm): 6.47 (1H, d,
J = 8.0 Hz), 6.71 (1H, d, J = 10.0 Hz), 7.19 (1H, t, J = 7.2 Hz), 7.66
(1H, d, J = 8.8 Hz), 8.18 (1H, m), 8.29 (1H, m), 8.42 (2H, m), 8.47
(4H, m), 8.57 (1H, m), 9.37 (1H, s), 10.81 (1H, s). QTOF-MS ES+
(Figure S2), m/z (M+ H)+, calculated for C24H16NO2: 350.1176,
335 nm; λex, 280 nm). Increasing naphthalene concentration to
the solution having fixed amount of ANB, APA and ANP enhance
their emission intensities, indicating FRET process. Weak emis-
sion of ANB (λem, 420 nm; 20 μM, Figure 2) gradually enhances
on concomitant addition of naphthalene (0.1–10 μM), attributed
to FRET from naphthalene to ANB. Interestingly, addition of
naphthalene over 10 μM does not alter the overlap area with
no further change of emission intensity. Reverse titration, where
gradual addition of ANB to fixed amount of naphthalene (Figure
S10, ESI) quenches naphthalene fluorescence because of energy
transfer from naphthalene to ANB. Plots of emission intensities
versus concentrations of naphthalene and ANB generate two
sigmoidal curves, the first one being growth (Figure S11, ESI),
and the later one is decay curve (Figure S12, ESI).
found, 350.1288. FTIR (neat) (Figure S3) νmax = 1587 and 1708 cmÀ1
.
On the other hand, APA (λem, 459 nm, λex, 280 nm) also par-
ticipates in the FRET process with naphthalene as reflected by
its fluorescence enhancement with increasing naphthalene
content. However, LOD of APA for naphthalene is lower than
that of ANB. It can detect as low as 1.0 μM naphthalene
(Figure 3). Plot of emission intensity of APA as a function of
naphthalene concentration is sigmoidal (Figure S13, ESI). Re-
verse titration by gradual addition of APA to a fixed amount
Scheme 1. Synthesis of the probes.
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TUNING FRET EFFICIENCY FOR VISIBLE REGION DETECTION OF NAPHTHALENE
Figure 3. Changes in the emission spectra of APA (20 μM, λex, 280 nm)
upon gradual addition of naphthalene (0, 1.0, 2.0, 3.0, 5.0, 10.0, 15.0, 20.0,
30.0, 50.0, 75.0, 100.0, 200.0, 500.0 and 1000.0 μM) in methanol–water
(4:1, v/v).
Figure 1. Overlap of emission of the donor (naphthalene, 50 μM) and
absorption of acceptors (ANP, APA and ANB, 20 μM) in methanol : water
(4:1, v/v).
Figure 4. Changes in the emission spectra of ANP (20 μM, λex, 280 nm)
in methanol–water (4:1, v/v) upon gradual addition of naphthalene (0,
0.01, 0.05, 0.1, 0.5, 1.0, 2.0, 3.0, 5.0, 10.0, 15.0, 20.0, 30.0, 50.0, 75.0,
100.0, 200.0, 300.0, 500.0 and 1000.0 μM).
Figure 2. Changes in the emission spectra of ANB (20 μM, λex, 280 nm)
in methanol : water (4:1, v/v) upon gradual addition of naphthalene (0,
0.1, 0.5, 1.0, 2.0, 3.0, 5.0, 10.0, 15.0, 20.0, 30.0, 50.0, 75.0 and 100.0 μM).
concentration. Reverse fluorescence titration for ANP (Figure 6)
shows quenching of naphthalene fluorescence upon increas-
ing ANP concentration, while Figure S17 (ESI) reveals the
corresponding hyperbolic decay curve. The quenching of
naphthalene fluorescence is highest for ANP compared with
ANB and APA.
of naphthalene (Figure S14, ESI) corroborates the said FRET
process. A sigmoidal decay curve is obtained when fluores-
cence intensity of naphthalene (50 μM) is plotted against
APA concentration (Figure S15, ESI).
Focusing to enhance the LOD for naphthalene, tuning of FRET
efficiency is necessary (overlap area of naphthalene donor emis-
Förster distances (Ro) between donor and acceptor in all three
cases have been calculated using the equation (Lakowicz, 2006),
Ro = 0.2108 [Κ2Φo À4J]1/6. The values of Ro for ANP, APA and
n
sion and absorbance of probe acceptor). LOD of ANP (λem
,
ANB are 27.77, 25.11 and 24.24 Å, respectively (Table S1, ESI).
Moreover, with decreasing (r/Ro),that is, with increasing Ro, en-
ergy transfer efficiency (E) increases. If it is assumed that the real
distance (r) between naphthalene and all three probes remain
the same, ANP will show the highest energy transfer efficiency.
Efficiency of energy transfer from naphthalene to the probes
has also been calculated using the equation ((Lakowicz),
E = 1 À (FDA/FD) where FDA and FD represent emission intensities
of donor in presence and absence of acceptor, respectively. The
calculated values in presence of naphthalene (1000.0 μM) are
0.860, 0.791 and 0.772 ANP, APA and ANB (20 μM), respectively.
459 nm; λex, 280 nm) for naphthalene is 0.01 μM. Because of
the presence of pyrene unit, it has higher absorbance at longer
wavelength than that of ANB (Figure 1), thus enabling larger
overlap area between naphthalene emission and ANP absor-
bance at higher naphthalene concentration (up to 1000 μM).
Figure 4 shows the fluorescence enhancement of ANP upon
gradual addition of naphthalene (from 0.01 μM to 1000.0 μM),
while Figure S16 (ESI) shows the related sigmoidal curve. Figure
S15 (ESI) has two linear regions, viz. 0.1 to 5 μM and 5 to 30 μM
(Figure 5), useful for determination of unknown naphthalene
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S. NANDI ET AL.
Figure 5. Linear plots: (a) 0.1 to 5 μM and (b) 5 to 30 μM of added naphthalene.
Figure 6. Changes of the emission spectra of naphthalene (50 μM,
ex = 280 nm) upon gradual addition of ANP (0, 0.01, 0.05, 0.1, 0.5, 1.0,
λ
2.0, 3.0, 5.0, 10.0, 15.0, 20.0, 30.0, 50.0, 75.0, 100.0, 200.0, 300.0, 400.0,
500.0 and 1000.0 μM).
Figure 7. Fluorescence lifetime decay of ANP (20 μM) and the (ANP
+ naphthalene) systems in methanol : water (4:1, v/v) medium (λex
,
280 nm and λem, 459 nm).
Common cations (Li+, Na+, K+, Ag+, Cd2+, Mg2+, Zn2+, Ca2+, Pb2+
,
Figure 1 clearly shows the higher overlap integral between
naphthalene emission (donor) and ANP absorption than that of
ANB and APA, indicating stronger FRET process involving ANP.
This allows ANP to detect much lower naphthalene concentra-
tion. Among most of the PAHs, only naphthalene may act as
an effective FRET donor for all the three probes. Naphthalene
having 335 nm emission is an ideal candidate for overlap with
the absorption of ANB (335 nm), APA (365 nm) and ANP
(360 nm), which is absent for other PAHs like dibenzo(a,h)anthra-
cene (λem, 404 nm), benzo(ghi)perylene (λem, 394 nm), indigo
(1,2,3 cd)pyrene (λem, 496 nm), benzo(a)pyrene (λem, 406 nm),
benzo(b) fluoranthene (λem, 446 nm), crycene (λem, 386 nm),
benzo(k)fluoranthene (λem, 414 nm), pyrene (λem, 406 nm),
fluoranthrene (λem, 460 nm) and anthracene (λem, 402 nm)
(Fernández-Sánchez et al., 2003)
However, APA and ANP have insignificant overlap with benzo
(ghi)perylene and crycene compared with naphthalene; how-
ever, they do not have excitation near 280 nm. Thus, the earlier
discussion clearly demonstrates that ANB, APA and ANP are
suitable for selective determination of naphthalene.
In presence of naphthalene, the fluorescence lifetime of ANP
(at 459 nm, acceptor emission) increases to 3.07 ns from that of
free ANP (2.23 ns), supporting the FRET process (Figure 7).
Co2+, Hg2+ and Cu2+) and anions (FÀ, ClÀ, BrÀ, IÀ, AcOÀ, NÀ3 ,
SCNÀ, ClOÀ4 , NO3À, SO42À and H2PO4À) normally present in water
do not show any significant interference (Figures S18 and S19).
Application
Gomati River, a tributary of the River Ganga, is one of the most pol-
luted rivers in India is affected during its course by three major ur-
ban centers, viz. Lucknow, Sultanpur and Jaunpur. Several groups
are working to measure the concentration of PAHs in this water.
The developed method is used to determine the contamination
of naphthalene in the Gomati river water. The method is also used
to determine percent recovery of naphthalene in Mili-Q water. To
evaluate the accuracy of the method, recovery studies have been
performed at different concentration levels of naphthalene. Stan-
dard addition method is used for determination of naphthalene
in water (Ellison and Thompson, 2008). Results obtained are sum-
marized in Tables S2 and S3 for Mili-Q and river water, respectively.
Table S3 indicates close resemblance of the present results to that
obtained using high-performance liquid chromatography method
(Malik et al., 2011), indicating the efficiency of ANP for naphthalene
determination.
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J. Mol. Recognit. 2016; 29: 303–307

TUNING FRET EFFICIENCY FOR VISIBLE REGION DETECTION OF NAPHTHALENE
CONCLUSION
Acknowledgements
Authors sincerely thank the Department of Environment,
Govt. of West Bengal for financial support. S. Nandi gratefully
acknowledges UGC for fellowship. Authors thank Dr. Bankim
Chandra Ghosh Haldia Government College, West Bengal, India,
for his help.
Three new low-cost fluorescence probes, viz. ANB, APA and ANP,
participate in the FRET process with naphthalene. FRET efficiency
is highest for ANP that allows lowest LOD for naphthalene. Fluo-
rescence lifetime data support the FRET process. Finally, ANP
has been successfully used to determine naphthalene concentra-
tion of Gomti river water, first time ever.
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Additional supporting information can be found in the online
version of this article at the publisher’s website.
J. Mol. Recognit. 2016; 29: 303–307
Copyright © 2016 John Wiley & Sons, Ltd.
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