A New, Highly Potent 1,8-Naphthalimide-based Fluorescence “Turn off” Chemosensor Capable of Cu2+ Detection in China's Yellow River Water Samples

JOURNAL OF THE CHINESE

CHEMICAL SOCIETY

Article

A New, Highly Potent 1,8-Naphthalimide-based Fluorescence “Turn off” Chemosensor

Capable of Cu²⁺Detection in China’s Yellow River Water Samples

*

Stephen Opeyemi Aderinto

School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, China

(Received: September 5, 2017; Accepted: October 25, 2017; DOI: 10.1002/jccs.201700308)

A new 1,8-naphthalimide-based ﬂuorescence “turn off” chemosensor, N-phenyl-4-(3,3⁰-((2-ami-

noethyl)azanediyl)dipropanoic acid)-1,8-naphthalimide (MAST), for the detection of Cu²⁺was

synthesized. Upon treatment with Cu²⁺, in coexistence with various competitive metal ions in

HEPES-buffered dimethylsulfoxide (DMSO) solution (v/v, 1:1; pH 7.4), MAST displayed a high

selectivity toward Cu²⁺with a ﬂuorescence quenching of 83.67%. Additionally, a good linear

response of MAST for the detection of Cu²⁺was obtained in the concentration range of 10 × 10⁻⁶

to 50 × 10⁻⁶M. A 1:1 stoichiometric interaction of MAST with Cu²⁺was observed, and the asso-

ciation constant and detection limit were calculated to be 1.37 × 10⁶and 0.69 × 10⁻⁸M, respec-

tively. The sensing mechanism of the chemosensor toward Cu²⁺was proposed due to the effect of

the paramagnetic nature of Cu²⁺and reverse-photo-induced electron transfer (PET) process. Ulti-

mately, the proposed chemosensor was applied to quantify Cu²⁺in real-world water samples, with

excellent recovery rates of 98.00–109.80% observed.

Keywords: 1,8-Naphthalimide; Fluorescence “turn off” chemosensor; Cu²⁺; Paramagnetic;

Reverse-photo-induced electron transfer process.

INTRODUCTION

Over the past few decades, massive efforts have

water at concentration levels of 31.5 and 20 μM, respec-

tively.^14,23Therefore, the selective monitoring of Cu²⁺

in both biological and environmental systems has

become a subject of much relevance in the ﬁeld of sens-

ing science.

been taken toward the constructions of chemosensors

for the monitoring of metal ions of biological and envi-

ronmental importance.^1–10In spite of the crucial roles

metal ions play in environmental and biological sys-

tems, their ecotoxicological impingements have consti-

tuted great challenges for a long time.^11,12More and

more attention is being focused on the construction of

chemosensors for Cu²⁺due to the critical roles it plays

by acting as a catalyst cofactor for some biologically

important proteins involved in growth and development

in the human body.^13–15Abnormal thresholds of Cu²⁺

homoeostasis in human bodies are usually linked with

some neurodegenerative diseases, including Wilson’s

disease, dyslexia, hypoglycemia, Alzheimer’s disease,

Menkes disease, Parkinson’s disease, amyotrophic lat-

eral sclerosis, infant liver damage, and gastrointestinal

disease.^16–22The toxicity data and scientiﬁc studies con-

ducted by the World Health Organization (WHO) and

US Environmental Protection Agency (US EPA) pro-

vided acceptable bearable limits of Cu²⁺in drinking

The early available methods of metal ion detection

include atomic absorption spectrometry, inductively

coupled plasma-atomic emission spectrometry, and

inductively coupled plasma-mass spectrometry. Despite

their merits, the constraints of difﬁculty, high cost of

equipment, and time-consuming and demanding proce-

dures still place some barriers on their applications.^24,25

Compared to these methods, the ﬂuorescence technique

has the superior features of high selectivity and sensitiv-

ity, simplicity of operation, low instrumentation cost,

and rapid analytical detection, thereby making it a

much better sensing technique.^26,27The 1,8-

naphthalimide and its derivatives have been widely

exploited as exquisite, sought-after ﬂuorophore moieties

for chemosensors’ construction due to their desirable

qualities of excellent photophysical ﬂuorescent proper-

ties of high photostability and quantum yield, strong

*Corresponding author. Email: 2014bsz528@stu.lzjtu.edu.cn

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Scheme 1. The synthetic path to the chemosensor MAST and reference compounds 2 and 3.

absorption band in the visible region, and large Stokes

shifts.^28–37Consequently, 1,8-naphthalimide derivatives

have been found to be great applications for the genera-

tion of liquid crystal displays, laser active media, ﬂuo-

rescent markers in biology, photo-induced electron

transfer (PET) chemosensors, electroluminescent mate-

rials, and ﬂuorescence switches and chemosensors.^38–42

To date, researchers from across the globe have per-

formed well with regard to the design and synthesis of

Cu²⁺-selective ﬂuorescence “turn off” chemosensors, with

some of those ﬁndings referenced,^43–52yet there are many

milestones uncovered in the science of the design and

development of Cu²⁺-selective chemosensors.

In this present communication, the synthesis of a

new variant of 1,8-naphthalimide that serves as a ﬂuores-

cence “turn off” chemosensor for the selective

detection of Cu²⁺, which employs the “ﬂuorophore-

spacer-receptor” convention, based on the PET

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1,8-Naphthalimide-based Fluorescence Chemosensor

anhydride was utilized as the initiating material. Interme-

diate compounds 2 and 3 were obtained in accordance

with the procedural steps reported by Misra et al.¹The

condensation of 4-bromo-1,8-naphthalic anhydride was

achieved using 1-phenylmethanamine in an ethanol

(EtOH) solvent to afford the intermediate compound 2,

which then underwent a nucleophilic aromatic substitu-

tion reaction upon the addition of ethylenediamine to

give the intermediate compound 3. The Michael addition

reaction of the intermediate compound 3 with methyl

acrylate in a methanol (MeOH) solvent yielded the inter-

mediate compound 4. Lastly, the desired chemosensor

MAST was afforded in quantitative yield by undergoing

acid hydrolysis. The full characterization of the chemo-

sensor was performed by Fourier transform infrared red

(FT-IR), ultraviolet–visible (UV–vis), proton nuclear

magnetic resonance (¹H NMR) and carbon-13 nuclear

magnetic resonance (¹³C NMR) spectroscopic methods

and MALDI-HRMS spectrometry (Figures S1–S4, Sup-

porting information). All the data obtained were in good

consistency with the structure of the proposed

chemosensor.

In an attempt to come up with a chemosensor

with an excellent solubility property, the carboxylic acid

group has been incorporated into the receptor moiety

of the newly derived compound, MAST, which is cur-

rently being reported. The choice of the carboxylic acid

functional group owes its stimulation and reason to the

fact that the ─COOH group readily displays a great

solubility in water. As such, while the chemosensor

MAST was being designed, the author speculated that

the incorporation of carboxylic acid groups into the

architectural framework of the chemosensor would lead

to its absolute solubility in an aqueous medium, which

is a very desirable quality many spectroscopic chemo-

sensors lack to date. Upon testing the solubility

Scheme 2. The structure of the chemosensor MAST,

showing the photo-induced electron trans-

fer (PET) model employed for its design

and the water-soluble carboxylic acid

functional group.

mechanism, is being reported.⁵³Interestingly, the che-

mosensor, which shows a signiﬁcant solubility in an

aqueous environment, also displays an ardent sensing

property toward Cu²⁺amidst the other metal ions tested

and can be used for the monitoring of Cu²⁺in the actual

environmental system, speciﬁcally in water samples.

RESULTS AND DISCUSSION

Synthesis and characterization

The design concept of the chemosensor MAST

stems from the “ﬂuorophore-spacer-receptor” paradigm

with a PET mechanism.⁵³The route to the synthesis of

the chemosensor MAST comprises a few steps, as explic-

itly illustrated in Scheme 1. 4-Bromo-1,8-naphthalic

Table 1. Photophysical properties of the chemosensor MAST (1 mM) in polar solvents buffered with HEPES (pH = 7.4)

Solvent

λ_F(nm)

λ_A(nm)

ε_max,10⁴(L/mol cm)

υ_A–υ_F(/cm)

Φ_F

DMSO

DMF

THF

532

523

506

527

528

516

517

444

440

429

443

440

429

430

3.905

7.530

5.183

2.637

4.650

4.087

3.751

3580.1

3387.9

3562.5

3705.0

4317.1

4429.7

3752.3

0.11

0.18

0.86

0.30

0.40

0.49

0.64

EtOH

MeOH

(CH₃)₂CO

CH₃CN

N = 3, RSD = 1.8%, where N = number of replicate determinations and RSD = relative standard deviation.

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property of the obtained compound, it was soluble in

water to a signiﬁcant extent, even though not abso-

lutely. MAST also displays complete and remarkable

solubilities in dimethylsulfoxide (DMSO), dimethylfor-

mamide (DMF), tetrahydrofuran (THF), EtOH,

MeOH, acetone (CH₃)₂CO, and acetonitrile (CH₃CN)

but total insolubility in dichloromethane and tetrachlor-

omethane. While Scheme 1 outlines the synthesis of

chemosensor MAST, Scheme 2 summarizes the design

module used for its obtainment.

Investigation of the photophysical properties

of chemosensor MAST

The fact that 1,8-naphthalimide derivatives owe their

absorption and ﬂuorescence characteristics to the nature of

the C-4 substituents on their rings is well established.^54,55

Table 1 presents the fundamental photophysical character-

istics (absorption maxima, λ_A; ﬂuorescence maxima, λ_F;

molar extinction coefﬁcient, ε_max; Stokes shift, υ_A–υ_F; and

quantum ﬂuorescence yield, Φ_F) of chemosensor MAST

obtained in seven solvents of varying polarities. The sol-

vents used for the assessment of the photophysical proper-

ties of chemosensor MAST were: DMSO, DMF, THF,

EtOH, MeOH, (CH₃)₂CO, and CH₃CN. The photophysi-

cal studies were conducted in these solvents as they provide

excellent solubilities for the chemosensor.

Notably,

derivatives

of

4-alkylamino-1,8-

naphthalimide are strong, yellow-greenish, ﬂuorescence-

emitting ﬂuorophores with maximal absorption bands

of about 430–440 nm in the blue region, which are

attributable to π–π* transitions. Under the naked eye,

chemosensor MAST displayed a yellow color in each of

these solvents, while it emitted a strong greenish ﬂuores-

cence under UV light. The obtained ﬂuorescence and

absorption bands were in the regions of 506–532 and

429–444 nm, respectively.

Increasing the solvent polarity usually leads to a

bathochromic shift of the maximum wavelength band.

It is, therefore, expected that DMSO, which has the

highest polarity, will display the highest values of the

absorption and ﬂuorescence wavelengths (i.e. λ_Aand

λ_F) and that THF, with the lowest polarity, will yield

the lowest values of λ_Aand λ_F. As speculated, these

results were obtained and strongly correlated with the

order of polarity. The range of the values of the molar

extinction coefﬁcient obtained is 26 370–75 300 L/mol

cm, which is representative of the usually high molar

extinction coefﬁcients of typical 1,8-naphthalimide

derivatives.

Stokes shift is a crucial photophysical parameter

of ﬂuorescent compounds that conveys the variations in

the properties and structure of the ﬂuorophores as they

switch between the ground state S₀and the ﬁrst excited

state S₁.⁵⁶Based on the equation below,⁵⁷the parame-

ters of the Stokes shift (cm⁻¹) fell within the range of

3387.9–4429.7 cm⁻¹, which is in concordance with the

data obtained for other blue-emitting 1,8-naphthalimide

derivatives⁵⁸:

Fig. 1. (a) pH effect on the ﬂuorescence spectra of

the chemosensor MAST (0.1 mM) in

HEPES-buffered DMSO solution; the pH

was modulated by adding 1 M HCl and/or

1 M NaOH in double-distilled water. (b) The

alterations at 529 nm with differing, increas-

ing pH values. (Note: Three sets of parallel

experiments were carried out, and the

approximate errors were estimated to be

about 3%).

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1,8-Naphthalimide-based Fluorescence Chemosensor

ꢀ

ꢁ

λ_F−λ_A

ðν_A−ν_FÞ =

× 10⁷

ð1Þ

λ_Aλ_F

The chemosensor MAST has large Stokes shift

values that are beneﬁcial as the excited and emitted

radiation can be separated with more efﬁciency. More-

over, this attests to the well-established fact that Stokes

shift is usually high in polar solvents, in which there is

the favorable formation of hydrogen bond and dipole

interactions, than in nonpolar solvents, in which there

is not.⁵⁹All the solvents used in this investigation are

polar, and the values of the Stokes shifts obtained from

them are commendable.

The ﬂuorescence quantum yield (Φ_F) quantiﬁably

typiﬁes and signiﬁes the potency of molecules to emit

absorbed light and is determined using the equation

below,⁶⁰with N-butyl-4-n-butylamino-naphthalimide

(Φ_F= 0.81 in EtOH) used as the standard:

Fig. 2. Fluorescence response of chemosensor MAST

(0.1 mM) to Cu²⁺(0.1 mM) or to a mixture

of each of the speciﬁed metal ions (0.1 mM)

with Cu²⁺(0.1 mM) in HEPES-buffered

DMSO solution (v/v, 1:1; pH 7.4). (Note:

Three sets of parallel experiments were car-

ried out, and the approximate errors were

estimated to be about 2.5%).

2

S_sA_rn_s

S_rA_sn_r

ϕ_F= ϕ_r

ð2Þ

2

protons in the vicinities of ﬂuorescent chemosensors

induces an inﬂuence on their sensing behaviors, which

in turn affect their sensitivities in the detection of an

analyte.

where Φ_Fstands for the sample’s emission quantum

yield; Φ_rdenotes the standard’s emission quantum

yield; A_rand A_sare the standard’s and sample’s absor-

bances at their respective excitation wavelengths; S_r

and S_sconnote the standard’s and sample’s integrated

emission band areas, respectively; and n_rand n_srepre-

sent the standard’s and sample’s solvent refractive

index, respectively.

The quantum yield decreases with increasing sol-

vent polarity, with THF having the highest value and

DMSO having the lowest. This is attributable princi-

pally to the PET quenching phenomenon. i.e. is usually

observed in nonpolar media, which successively ensued

in the regeneration of the ﬂuorescence emission, and the

PET processes that are accelerated in polar solvents.⁶¹

With this in mind, the ﬂuorescence spectra of the

chemosensor MAST was monitored as a function of

variation in pH by conducting pH titration of the che-

mosensor MAST with solutions of different pH values.

To acquire a great depth of insight, a wide pH range of

0.54–13.58 was utilized, with the pH modulated incre-

mentally with 1 M HCl and/or 1 M NaOH, starting

from the acidic region. As shown in Figure 1(a), upon

acidiﬁcation, large ﬂuorescence enhancements were

observed in the starting pH range of 0.54–1.53, with the

implication that the ﬂuorescence emission is highly

dependent on the pH in the acidic region. This is

accrued to the protonation of the aniline nitrogen on

the naphthalimide ring,³which in turn weakened its

interaction with Cu²⁺, thereby leading to enhanced

ﬂuorescence intensity (“turned on” ﬂuorescence). More

interestingly, only minuscule changes (less than 2.46%

variation) in the ﬂuorescence intensity of the chemosen-

sor occurred in the pH region 1.53–8.62. Simply put,

the determination of Cu²⁺with the chemosensor MAST

was not signiﬁcantly affected by variation in pH in this

region. This is, therefore, proof that the synthesized

Investigation of the effect of pH on the sensing properties

of chemosensor MAST

The next logical step would be to gain an under-

standing of the inﬂuence of operating pH on the sensing

properties of chemosensor MAST. This will, in turn,

inform the determination of the pH at which the pro-

posed chemosensor performs best, which will largely

inﬂuence the sensing characteristics of the chemosensor.

It is generally known that the presence or absence of

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Fig. 3. Fluorescence spectra of the chemosensor

Fig. 4. Job plot for the complex [Cu(MAST)] (the

MAST (0.1 mM) in HEPES-buffered

DMSO solution (v/v, 1:1; pH 7.4) in the

joint existence of differing quantities of com-

peting metal ions (0.1 mM). (Note: Three

sets of parallel experiments were carried out,

and the approximate errors were estimated

to be about 2.7%).

entire concentration of MAST and Cu²⁺was

held constant at 100 μM). (Note: Three sets

of parallel experiments were performed, and

the approximate errors were estimated to be

approximately 4.6%).

a 0.1 mM HEPES-buffered DMSO solution (v/v, 1:1;

pH 7.4), only Cu²⁺was able to quench the ﬂuorescence

of the chemosensor. This spectacular observation

occurred instantly, affording an 83.67% ﬂuorescence

decrease or 6.12-fold ﬂuorescence quenching (Φ_F=

0.018). This observation is ascribable to the coordination

of the powerful paramagnetic Cu²⁺to the receptor unit of

the chemosensor, which effected a ﬂuorescence “turn off”

behavior when the solution of each of the metal ions was

added.^48–52In sharp contrast, each of the other metal ions

investigated under the same conditions had a collective

benign effect (a persistent greenish color under UV light)

on the ﬂuorescence intensity of the chemosensor. These

outcomes clearly furnish the evidence that the chemosen-

sor MAST exhibits both singular and ardent signaling

behavior toward Cu²⁺and could effectively discriminate

Cu²⁺in an admixture of several other metal ions. More-

over, this commendable result validates the soundness of

our design strategy of the chemosensor MAST.

chemosensor could facilitate the monitoring of Cu²⁺in

a wide pH span, and can behave as a functional chemo-

sensor in low- and medium-pH value media. Based on

experimental results as justiﬁed in Figure 1(b), pH 7.4

was chosen as the optimum pH value at which the pro-

posed chemosensor works best under experimental

conditions.

Conversely, the ﬂuorescence intensity of the che-

mosensor suffered a marked decrease (“turned off”

ﬂuorescence) in the pH region 9.58–13.58. This is attrib-

uted to the deprotonation of the hydroxyl functional

group in the receptor moiety, which in turn led to an

extension of the conjugation, and thereby reduced the

ﬂuorescence intensity.

Spectroscopic assessment of chemosensor MAST in the

presence of various metal ions

The ﬂuorescence response of the chemosensor

MAST to various metal ions, i.e. Na⁺, K⁺, Ca²⁺, Mg²⁺,

Al³⁺, Pb²⁺, Fe³⁺, Ni²⁺, Zn²⁺, Hg⁺, Ag⁺, Co²⁺, Cr²⁺,

Mn²⁺, Cd²⁺, and Cu²⁺in HEPES-buffered DMSO solu-

tion (v/v, 1:1; pH 7.4) under excitation at 435 nm was

investigated. In the absence of the metal ions, the chemo-

sensor exhibited large ﬂuorescence strength (an intensive

greenish ﬂuorescence) at 529 nm. As Figure 2 clearly

depicts, when each of the metal ions was introduced into

Selectivity study of MAST for Cu²⁺over competitive

metal ions

Whether a proposed compound will act as an ideal

chemosensor or not is largely a matter of its selectivity.

As such, the selectivity property of the proposed che-

mosensor was studied by looking at the effects of other

metal ions on its sensing properties. Drawing on this,

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1,8-Naphthalimide-based Fluorescence Chemosensor

and Cu²⁺, which were also at concentrations of

0.1 mM. Figure 3 demonstrates the plot of the ratios of

the ﬂuorescence intensities versus the wavelength and

vividly indicates that the competition between Cu²⁺and

other metal ions for the dampening of the ﬂuorescence

chemosensor MAST is modest. Meanwhile, Cu²⁺is the

only ion that signiﬁcantly suppressed the ﬂuorescence

intensity of the chemosensor MAST. Likewise, the

emission color is independent of the metal ion added,

except for Cu²⁺ion, which induced a near-colorless

effect on the solution of the chemosensor. In short, the

addition of each of the metal ions does not impose a

prominent disturbance on the ﬂuorescence of the [Cu

(MAST)] complex. These results strongly correlated

with those acquired in the previous section. This clearly

demonstrates that the selectivity of the proposed che-

mosensor is excellently high and that the chemosensor

could retain its sensing properties toward Cu²⁺under

the potential competition of other metal ions. This is a

further attestation to the soundness of the proper choice

of entities used in the construction of the

chemosensor MAST.

Fig. 5. Benesi–Hildebrand plot for the estimation of

the association constant of complex [Cu

(MAST)], using a 1:1 stoichiometry for the

interaction between MAST and Cu²⁺. (Note:

Three sets of parallel experiments were car-

ried out, and the approximate errors were up

to 4%).

ﬂuorescence competitive experiments were conducted in

the presence of Cu²⁺at a concentration of 0.1 mM

mixed with each of the various interfering metal

ions, speciﬁcally Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, Pb²⁺

,

Fe³⁺, Ni²⁺, Zn²⁺, Hg⁺, Ag⁺, Co²⁺, Cr²⁺, Mn²⁺, Cd²⁺

Determination of the stoichiometric ratio and binding

constant

The method of continuous variation, i.e. Job

plot,⁶²was used to obtain the coordination stoichiome-

try of the complex formed, [Cu(MAST)], between the

chemosensor MAST and Cu²⁺. The maxima at the

mole fraction at 0.5 of the Job plot (Figure 4) clearly

reveals the formation of a 1:1 complex between the che-

mosensor MAST and Cu²⁺ion.

The association constant and detection limit

(DL) of the chemosensor MAST were determined by

employing the 1:1 stoichiometry. First, the association

constant (K_a) of the chemosensor MAST was estimated

using the Benesi–Hildebrand equation⁶³:

1

=

+

ð3Þ

Â

Ã

n

K_aðF_min−F₀Þ Cu^{2 +}

F −F₀

F

_min−F₀

Fig. 6. The Stern–Volmer plot of the ratio of the

ﬂuorescence intensities F₀/F (where F₀is the

blank’s ﬂuorescence intensity, which is 2191)

of the chemosensor MAST against the

increasing concentration of Cu²⁺([Cu²

⁺] = 10–100 μM). (Note: Three sets of paral-

lel experiments were carried out, and the

approximate errors were estimated to be

about 3%).

where F is the ﬂuorescence intensity at 529 nm at any

given concentration of Cu²⁺, F₀is the ﬂuorescence

intensity at 529 nm in the absence of Cu²⁺, F_minis the

minimum ﬂuorescence intensity at 529 nm in the pres-

ence of Cu²⁺in solution, and n is the stoichiometric

mole ratio, which is 1 in this case. The association

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constant was appraised by graphically plotting 1/(F −

F₀) against 1/[Cu²⁺] (Figure 5). Data were subjected to

linear ﬁts according to the above equation, and the K_a

value was determined from the line’s slope. The value

of the association constant was calculated to be 1.37

(ꢀ0.04) × 10⁶M⁻¹(the calculated percentage errors

were less than 3%), which is large enough and indicates

an intense binding of the chemosensor MAST with the

Cu²⁺ion. This value correlates with those obtained for

other Cu²⁺chemosensors.^64–67

Determination of DL of chemosensor MAST

The ﬂuorescence quenching for the “turn off” phe-

nomenon was evaluated quantitatively by observing the

Stern–Volmer plot of the ratio of the ﬂuorescence inten-

sities versus the concentration of Cu²⁺(Figure 6) based

on the equation below⁶⁸:

F₀

F

= 1 + K_sv½Qꢁ

ð4Þ

where F₀is the initial ﬂuorescence intensity; F is the

ﬂuorescence intensity measured in the presence of the

analyte; [Q] is the concentration of the quencher, which

is [Cu²⁺] in this case; and K_svis the Stern–Volmer con-

stant. When (F₀/F) is plotted against [Q], the plot was a

straight line graph, with K_svbeing equivalent to the

value of the slope.

The DL,⁶⁹which is also a distinctive feature any

proposed compound must possess to qualify as an ideal

chemosensor, was equally obtained. The DL of chemo-

sensor MAST toward Cu²⁺was calculated based on the

equation below:

3σ

K_sv

DL =

ð5Þ

where σ is the blank solution’s standard deviation, and K_sv

is the calibration plot’s slope. DL was calculated to be 0.69

(ꢀ0.03) × 10⁻⁸M (the calculated percentage errors were

about 3%), which is far lower than the WHO and US EPA

regulated limits of 31.5 × 10⁻⁶and 20 × 10⁻⁶M,

respectively,^14,23for Cu²⁺in drinking water. This result

substantiates that the proposed chemosensor MAST has

the potency to monitor Cu²⁺in acceptably low concentra-

tions whether in the environmental or biological system.

All the experiments above yielded robust results,

and the aforementioned ﬁndings ﬁt exactly with the

focus of this present communication—the synthesis of a

Cu²⁺-selective ﬂuorescence “turn off” chemosensor. It

can, therefore, be argued that the proposed chemosen-

sor MAST meets the criteria for the highly selective

Cu²⁺sensing in a competitive medium of various physi-

ologically and environmentally relevant metal ions.

Fig. 7. (a) Absorption spectra response of MAST

(0.1 mM) in HEPES-buffered DMSO solu-

tion (v/v, 1:1; pH 7.4) upon the addition of

different concentrations of Cu²⁺(0.04 mM).

(b) Changes in absorbance at 438 nm with

differing, increasing Cu²⁺concentrations.

(Note: Three sets of parallel experiments

were carried out, and the approximate errors

were estimated to be about 3%).

UV–Vis and ﬂuorescence titration of chemosensor

MAST toward Cu²⁺

To investigate the binding afﬁnity of the chemo-

sensor MAST with Cu²⁺, UV–vis and ﬂuorescence

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Scheme 3. The proposed binding mode of the che-

mosensor MAST with Cu²⁺. The strong

greenish ﬂuorescence was turned off by

Cu²⁺in HEPES-buffered DMSO solution

(v/v, 1:1; pH 7.4). Inset: the initially

strong and consequently dampened ﬂuo-

rescence emission colors of the designated

compound MAST.

details of the titration studies conducted are given

below.

Fig. 8. (a) Changes in the ﬂuorescent spectra of the

chemosensor MAST (0.1 mM) in HEPES-

buffered DMSO solution (v/v, 1:1; pH 7.4)

upon the addition of different Cu²⁺concen-

trations (0.04 mM). (b) The changes in the

ﬂuorescence intensity at 529 nm with differ-

ing, increasing Cu²⁺concentrations (excita-

tion was performed at 435 nm). (Note: Three

sets of parallel experiments were carried out,

and the approximate errors were estimated to

be about 3.1%).

UV–vis titration studies

Figure 7(a) provides the UV–

vis absorption titration proﬁle of the chemosensor

MAST with varying concentrations of Cu²⁺solution.

When the concentration of Cu²⁺was increased (the

increment was by a factor of 10, starting from 10 up to

100 μM), the intensity of the main absorption peak at

438 nm underwent an apparent gradual decrease, fol-

lowed by the concomitant formation of a new signal

band positioned at 437 nm until a well-deﬁned isosbes-

tic point at 429 nm was formed. Moreover, the dose-

dependent absorbance quenching of the chemosensor

toward Cu²⁺, as demonstrated in Figure 7(b), exhibited

a good linearity even up to when 50 μM of the Cu²⁺

solution was added.

titration experiments were conducted. The spectral

behavior of the chemosensor MAST (0.1 mM) in

HEPES-buffered DMSO solution (v/v, 1:1; pH 7.4) was

recorded. The uncomplexed chemosensor displayed a

profound absorption band centered at λ_max= 438 nm

(ε_max= 1.4093 × 10⁵L/mol cm), which is responsible

for its sharp yellow color in solution. The chemosensor

also emits an intensive greenish ﬂuorescence at λ_F=

529 nm (Φ = 0.107), which is a representative feature

of the ﬂuorescence quantum yields of some ﬂuorescent

and 4-amino-1,8-naphthalimide compounds.^70–73The

Fluorescence titration studies

Utilizing experimental

procedures similar to those of UV–vis titration, further

insights into the sensing properties of the chemosensor

MAST were obtained by investigating the ﬂuorescence

titration reaction curve of the changes in the

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ﬂuorescence spectra of the chemosensor toward varying

amounts of Cu²⁺(Figure 8(a)). The ﬂuorescence titra-

tion of the chemosensor MAST with Cu²⁺experiment

reveals that the ﬂuorescence intensity of the chemosen-

sor was quenched in a steady manner in response to

the gradual increase in the concentrations of the added

Cu²⁺in HEPES buffered-DMSO solution (v/v, 1:1;

pH 7.4), with a bathochromism of about 3 nm

observed. This could be explained on the basis of the

complexation of Cu²⁺, which lowers the electron-

releasing potency of the nitrogen atom in the receptor

moiety,⁷⁴thereby inhibiting the PET mechanism and

the paramagnetic property of Cu²⁺. This observed phe-

nomenon of ﬂuorescence quenching indicates that the

chemosensor can effectively act as a “turn off” chemo-

sensor toward Cu²⁺. As clearly depicted in Figure 8(b),

the dose-dependent ﬂuorescence quenching of the che-

mosensor toward Cu²⁺showed a good linearity even up

to when 50 μM of the Cu²⁺solution was added so that

50 μM could be set as the threshold for the determina-

tion of Cu²⁺. This is an encouraging outcome, which

establishes the effectiveness of the proposed chemosen-

sor to detect quantitatively relevant concentrations of

between the ﬂuorescence moiety and the electron dona-

tion unit.⁷⁵In the chemosensor MAST, an aliphatic sec-

ondary amine connects to the naphthalimide

ﬂuorophore. In an alkaline medium, an aliphatic amine

usually acts as a powerful electron-releasing group.

Under this condition, the intensity of the ﬂuorescence

of the chemosensor MAST exists in the “off state” due

to the quenching of the intensity of the ﬂuorescence of

the naphthalimide ﬂuorophore by the PET process. On

the other hand, the binding mode is also partly the

result of the strong, intrinsic paramagnetic behavior of

the Cu²⁺ion.^48–52

Real-world application of chemosensor MAST for Cu²⁺

quantiﬁcation

The designated chemosensor MAST was used to

determine the amounts of Cu²⁺in real water samples.

The water samples employed for the analysis were

sourced from the famous Yellow River (Lanzhou city),

while the tap water samples were acquired from our

school. The samples were suitably spiked with the stan-

dard Cu²⁺solutions at various levels of concentrations.

Thereafter, they were analyzed with the chemosensor

MAST. The measurements were performed at three

(N = 3) replicate determinations for each of the Yellow

River and tap water samples. As depicted in the table,

the relative standard deviation (RSD) at the Cu²⁺sam-

ple concentrations were determined to be sufﬁciently

low (within the range of 0.02–0.30 μM). The recoveries

of the spiked Cu²⁺were calculated and summarized in

Table 2. The results of the analysis demonstrated recov-

eries in the range of 98–109.80%, meaning that the

chemosensor actually has potential for the detection of

Cu²⁺in actual samples.

Cu²⁺

.

Proposed sensing mechanism

The chemosensor MAST was designed as a typical

example of PET sensors that exploit the general

ﬂuorophore-spacer-receptor format.⁵³Scheme 3 pro-

vides a detailed schematic illustration of the proposed

binding mode. The binding mode of the chemosensor

MAST toward the Cu²⁺ion, which resulted in a

remarkable dampening of the intensity of the ﬂuores-

cence spectrum, is due to two plausible reasons. On the

one hand, when a ﬂuorescence group is joined to an

electron-donating group (in this case, the amido nitro-

gen atom), there is a transduction of the PET effect

Table 2. Results of the analysis of Cu²⁺ion determination in different water samples using the proposed chemosensor MAST

Water sample

Cu²⁺spiked (μM)

Cu²⁺recovery, mean ꢀ SD (μM)

Recovery (%)

Yellow

River

2

3

4

2

3

4

2.01 ꢀ 0.11

3.12 ꢀ 0.22

3.96 ꢀ 0.14

1.89 ꢀ 0.02

3.05 ꢀ 0.23

4.23 ꢀ 0.30

101.00

105.62

98.75

98.00

103.30

109.80

Tap

N = 3, where N = number of replicate determinations.

10

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JOURNAL OF THE CHINESE

CHEMICAL SOCIETY

1,8-Naphthalimide-based Fluorescence Chemosensor

EXPERIMENTAL

Procedures for spectroscopic measurements

Reagents and materials

The cations’ stock solutions (1 mM) were pre-

pared from the metal salts NaNO₃, KNO₃, CaCl₂,

MgCl₂, Al(NO)₃, Pb(NO₃)₂, Fe(NO₃)₃, Ni(NO₃)₂,

Zn(NO₃)₂, Cu(NO₃)₂, Hg(NO₃)₂, AgNO₃, Co(NO₃)₂,

Cr(NO₃)₃, Mn(NO₃)₂, and Cd(NO₃)₂. Test solutions of

the metal ions were prepared using double-distilled

water by diluting the stock solutions to the desired

molar concentration (0.1 mM). Similarly, the stock

solution (1 mM) of the chemosensor MAST was pre-

pared in the DMSO solvent, while the test solution of

the chemosensor was obtained by diluting the stock

solution to the desired molar concentration (0.1 mM).

The absorption and ﬂuorescence spectra of the solution

of the chemosensor cation were recorded using the mix-

ture of 2 mL of HEPES buffer solution, 20 μL of sam-

ple solution of the chemosensor, and 100 μL of each of

the various metal ions in a 1 cm × 1 cm quartz cuvette.

The mixture was allowed to stand for 1 min for thor-

ough mixing. The ﬂuorescence measurements were con-

ducted at the 435 nm maximum excitation wavelength

and the 750 nm maximum emission wavelength, while

the absorbance measurements were carried out at the

wavelength range of 180–550 nm. All the spectroscopic

experiments were performed in triplicates (i.e. N = 3).

The grade of the solvents and reagents used for

the syntheses were analytical reagent, and the materials

were utilized just as bought without any further puriﬁ-

cation performed, except if stipulated. Twice-distilled

water was used in all syntheses, while deionized water

was used for all spectroscopic measurements. Sixteen

nitrate and chloride metal salts were used as the sources

of the metal ions. Speciﬁcally, they are NaNO₃, KNO₃,

CaCl₂, MgCl₂, Al(NO)₃, Pb(NO₃)₂, Fe(NO₃)₃,

Ni(NO₃)₂, Zn(NO₃)₂, Cu(NO₃)₂, Hg(NO₃)₂, AgNO₃,

Co(NO₃)₂, Cr(NO₃)₃, Mn(NO₃)₂, and Cd(NO₃)₂.

HEPES (pH = 7.4) was used as the buffer solution. All

experiments were performed at room temperature.

Instrumentation

The progress of all reactions was periodically

monitored by thin-layer chromatography (TLC) per-

formed on silica gel, Fluka F60 254, 20 × 20, 0.2 mm

(Merck KGaA, Germany). Melting points were deter-

mined with XD-4 digital micromelting point apparatus

(Beijing, China) and were uncorrected. The Fourier

transform infrared (FT-IR) spectra were recorded in the

4000–400 cm⁻¹region with a Varian Scimitar 1000

spectrometer (Madison, WI, USA) using KBr pellets.

Electrospray ionization mass spectra (ESI-MS) were

acquired using a BRUKER micrOTOF-Q system

(Kontich, Belgium). UV–vis spectra were assessed using

a LabTech BlueStar Plus UV–vis spectrophotometer

(Tianjin, China). Fluorescence spectra were measured

using a Lengguang Tech. F97 Pro spectroﬂuorometer

(Shanghai, China). All the absorption and ﬂuorescence

spectra were recorded at 25^ꢂC in a 1 cm × 1 cm quartz

cuvette (3 mL) utilizing a 10 nm bandwidth and a

Synthesis of compounds

The proposed ﬂuorescent chemosensor MAST was

devised through a stepwise synthetic route of four steps

as outlined in Scheme 1. The starting point of the syn-

thesis was 4-bromo-1,8-naphthalic anhydride (com-

pound 1). Intermediate compounds 2 and 3 were

synthesized by the slight modiﬁcation of the procedures

earlier reported by Misra et al.¹

Synthesis of intermediate compound 2

The compound

1,4-bromo-1,8-naphthalic anhydride (8.30 g, 0.03 mol)

dissolved in 80 mL EtOH was added to 1-

phenylmethanamine (5 mL, 0.05 mol) in 20 mL of

EtOH. The resultant mixture was heated under reﬂux

1

3000 nm/min scan rate. The spectra of H NMR and

¹³C NMR were recorded on a Varian Mercury Plus

400 spectrometers (Palo Alto, CA, USA), with tetra-

methylsilane (TMS) as the internal standard and

DMSO-d₆as the solvent. Data, graphs, and chemical

structures were analyzed, plotted, and drawn with

Microsoft Excel, Origin 8.5, Mnova NMR, ChemDraw

Ultra 12.0, and Omnic™ software. The pHs of the cat-

ion solutions used were adjusted using a Beckman

Coulter F360 pH meter (Brea, CA, USA) utilizing M

HCl and/or 1 M NaOH.

ꢂ

and stirred for 4 h at 70 C. The progress of the reac-

tion was monitored by TLC analysis using ethyl ace-

tate/hexane (v/v, 2:8) as the eluent. After the complete

reaction was ascertained, 250 mL of cold water was

added to the reaction mixture to facilitate the precipita-

tion of the desired product. The precipitated product

was then isolated by vacuum ﬁltration and washed with

10% aqueous Na₂CO₃and distilled water. The product

J. Chin. Chem. Soc. 2017

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11

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Aderinto

was ﬁnally dried and obtained as an off-white solid;

yield: 89.71%, M.p.: 140–142^ꢂC.

and 2592 (νN); 2871 and 2960 (νCH); 1684

(ν^asN─C═O); 1646 (ν^sN─C═O); 1717 (νC═O of

COOH); 3663 and 3669 (νOH of COOH); 1550 and

1582 (νC═C, aromatic); 1300 and 1350 (νC─O of

Synthesis of intermediate compound 3

The intermediate

compound 2 (1.00 g, 0.0027 mol) was added to a solu-

tion of ethylenediamine (11.00 g, 0.183 mol) in a two-

necked 500 mL, round-bottom ﬂask, which was heated

to a temperature of 65^ꢂC on a reﬂux apparatus. The

resultant mixture was heated to reﬂux for 4 h with thor-

1

COOH). UV–vis (in DMSO, nm): 259, 445. H NMR

(300 MHz, DMSO-d₆): δ = 8.57 (d, 1H, J = 7.3 Hz,

naphthalimide H-5); 8.45 (d, ¹H, J = 8.6 Hz, naphthali-

mide H-2); 8.38 (dd, 1H, J = 8.3 Hz, J = 1.0 Hz,

naphthalimide H-7); 7.60 (dd, ¹H, J = 8.3 Hz,

ꢂ

ough stirring at 65 C. TLC monitoring of the reaction

1

J = 7.3 Hz, naphthalimide H-6); 6.67 (d, H = 8.6 Hz,

was performed at an hourly interval using 2 mL of

MeOH as the eluent. After the reaction was complete,

the reaction mixture was cooled to room temperature;

100 mL of distilled water was then poured into the

solution, whereupon the precipitate formed was col-

lected by suction ﬁltration, washed with distilled water,

and dried under vacuum. The crude product was

afforded in 80% yield. The puriﬁcation of the crude

product was performed by column chromatography on

a silica gel stationary phase with MeOH as the eluent;

yield: 75.67%; M.p.: 169–170^ꢂC.

naphthalimide H-3); 6.28 (m, ¹H, NH); 4.16 (t, 2H,

J = 7.6 Hz, (CO)₂NCH₂); 11.0 (s, 2H, 2 × OH); 2.39

(t, 4H, J = 6.4 Hz, 2 × CH₂COOH); 7.54 (d, 2H,

J = 7.2 Hz, benzene), 7.25 (m, 3H, benzene). ¹³C NMR

(400 MHz, DMSO-d₆): 176.64, 175.04, 168.80, 167.97,

154.96, 143.02, 139.20, 135.97, 134.24, 133.40, 132.73,

132.04, 129.58, 126.67, 125.47, 113.68, 109.38, 65.72,

56.02, 53.31, 53.14–52.66, 47.65, 45.31, 45.13, 44.71,

44.55–44.53, 44.40, 44.22, 44.09, 42.74, 33.44, 19.07.

Synthesis of intermediate compound 4

A solution of

CONCLUSIONS

methyl acrylate (2.50 g, 0.029 mol) in 10 mL of MeOH

was added slowly to a solution of intermediate com-

pound 3 (1.00 g, 0.0029 mol) in 60 mL of MeOH via a

syringe for a 30-min time frame. Thereafter, the reac-

tion mixture was continuously stirred for a 5-day time

frame at room temperature, the surplus methyl acrylate

was collected by suction ﬁltration, and the intermediate

product 4 was afforded as a solid of yellow color and

then dried under vacuum; yield: 81%; M.p.: 178–179^ꢂC.

In summary, we have successfully constructed a

new, highly potent 1,8-naphthalimide-based ﬂuores-

cence “turn off” chemosensor, MAST and evaluated its

sensing properties toward Cu²⁺. The compound yielded

a signiﬁcant solubility in water and in a host of other

solvents. Through a dampened ﬂuorescence mechanism,

the chemosensor upheld a strict monopoly for the sens-

ing of Cu²⁺in HEPES-buffered DMSO solution (v/v,

1:1; pH 7.4) in the presence of the other competitive

metal ions, such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, Pb²⁺

Fe³⁺, Ni²⁺, Zn²⁺, Hg⁺, Ag⁺, Co²⁺, Cr²⁺, Mn²⁺, Cd²⁺

,

Synthesis and characterization of the proposed chemosensor N-

phenyl-4-(3,3⁰-((2-aminoethyl)azanediyl)dipropanoic acid)-1,8-

and Cu²⁺. The DL of the chemosensor for the monitor-

ing of Cu²⁺reached the level of 1.69 × 10⁻⁸M, which

was lower than the WHO and US EPA limits of 31.5

and 20 μM, respectively, thereby opening up inspiration

for the deﬁnitive role of the chemosensor for the practi-

naphthalimide (MAST)

The desired compound

MAST was obtained in a quantitative yield by the

hydrolysis of intermediate compound 4 with potassium

hydroxide in MeOH solvent under reﬂux conditions for

2–3 h at a temperature of 75^ꢂC, followed by acidiﬁca-

tion by the drop-wise addition of 1 M HCl solution

until the pH reached 2–3. Once this pH range was

reached, a red precipitate of the desired product was

formed, which was then collected by vacuum ﬁltration

and washed thoroughly with water; yield: 85%; M.p.:

170–174^ꢂC. The crude product was ﬁnally recrystallized

in EtOH to afford the pure product; yield: 77.19%; M.

p.: 167–168^ꢂC. MS (ES⁺) m/z = 490.1853 (M + H)⁺.

FT-IR (v_max, KBr, cm⁻¹): 757 (ν_Ar); 3381 (νNH); 2603

cal quantitative analysis of Cu²⁺

.

It is apparent that the proposed chemosensor ﬁts

the description of an excellent example of Cu²⁺-selective

ﬂuorescence “turn off” chemosensors. Presently, the

research team to which the author belongs is actively

working on the generation of newer architectures of 1,8-

naphthalimide-based ﬂuorescent chemosensors for other

ions. With devoted efforts, I look forward to reporting

the future research investigations in due course.

12

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J. Chin. Chem. Soc. 2017

JOURNAL OF THE CHINESE

CHEMICAL SOCIETY

1,8-Naphthalimide-based Fluorescence Chemosensor

23. H. S. Jung, P. S. Kwon, J. W. Lee, J. Kim II. ,

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This work was supported by the China Scholar-

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J. Chin. Chem. Soc. 2017

Article Doi

A New, Highly Potent 1,8-Naphthalimide-based Fluorescence “Turn off” Chemosensor Capable of Cu2+ Detection in China's Yellow River Water Samples

DOI: 10.1002/jccs.201700308

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Article abstract of DOI:10.1002/jccs.201700308

Full text of DOI:10.1002/jccs.201700308

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