Rhodamine-derived highly sensitive and selective colorimetric and off–on optical chemosensors for Cr3+

Accepted Manuscript Title: Rhodamine-derived highly sensitive and selective colorimetric and off-on optical chemosensors for Cr3+ Author: Vinod Kumar Gupta Naveen Mergu Ashok Kumar Singh PII: DOI: Reference:

S0925-4005(15)00694-2 http://dx.doi.org/doi:10.1016/j.snb.2015.05.075 SNB 18516

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

4-5-2015 19-5-2015 19-5-2015

Please cite this article as: V.K. Gupta, N. Mergu, A.K. Singh, Rhodamine-derived highly sensitive and selective colorimetric and off-on optical chemosensors for Cr3+ , Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.05.075 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

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Transformations of chemodosimeter in its various chemical environments

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Two probes have been prepared and characterized by IR, 1H NMR, 

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C NMR and ESI-

MS.

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Highlights

Highly selective and sensitive chemosensor towards Cr3+ ions over other metal ions. 

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1:1 stoichiometry of probe to Cr3+ was confirmed by ESI-MS. 

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Exhibited a good binding constant and lowest detection limit towards Cr(III). 

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Serves as reversible chemosensor for Cr3+ by using a strong chelator such as EDTA. 

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Rhodamine-derived highly sensitive and selective colorimetric and off-on

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optical chemosensors for Cr3+

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Vinod Kumar Gupta*a,b,c, Naveen Mergua, Ashok Kumar Singha

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Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247 667, India Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

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Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa

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*Corresponding author. Tel.: +91 1332285801; fax: +91 1332273560.

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E-mail addresses: [email protected], [email protected] (V.K. Gupta).

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Abstract Two novel fluorescent rhodamine derivatives L1 and L2 have been synthesized and

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characterized by various analytical techniques. The sensors exhibited an extremely selective

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and sensitive “turn-ON” fluorescent and colorimetric response toward Cr3+ in methanol.

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Upon the addition of Cr3+, the spirolactam ring of probes was opened and a 1:1 metal–ligand

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complex was formed. The stoichiometry between Cr3+ and probe was further clarified by

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mass spectra. The chemodosimeter (L1 and L2) exhibited a good binding constant and lowest

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detection limit towards Cr(III), and also successfully examined the reversibility of

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complexation of metal to ligand (opened ring to spirolactam ring).

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Keywords: Chemosensor, Rhodamine, Optical, Colorimetric, Recognition

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1. Introduction Trivalent chromium, Cr(III), is a necessary metal ion of a balanced human and animal

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diet. Chromium is used in metal finishing, electroplating, chromate preparation and leather

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tanning processes. Chromium is a famous environmental contaminant that accumulates

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because of industrial and agricultural activities [1], and causes epigastric pain, hemorrhage,

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severe diarrhea and carcinogen effect [2]. Also its deficiency can increase the risk factors

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related with cardiovascular and diabetes diseases including elevated circulating insulin,

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triglycerides, total cholesterol, lipid metabolism and glucose levels [3,4].

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Thus, selective detection of such harmful metal ions at the sub-milli and micromolar

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level for environmental, clinical and biological purposes is highly attractive and essential.

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Even though various analytical methods, such as inductively coupled plasma mass

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spectroscopy, atomic absorption and emission spectrometry, chromatography, neutron

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activation analysis, X-ray fluorescence spectrophotometry, anodic stripping voltammetry and

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others [5–19], etc., have been played a role to detect these metal ions.

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Among several detection methods for metal ions, the colorimetric and fluorometric

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methods have become more useful and popular in medicine, biology and environmental

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chemistry due to its non-destructive, high selective and sensitive, quick and naked eye

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detection [20–33]. The rhodamine moiety to construct OFF-ON fluorescent chemosensors is

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a reliable method due to their structure change from non-fluorescent spirolactam to highly

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fluorescent ring-opened amide induced by specific chemical species at room temperature

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[34,35]. Rhodamine derivatives are used widely as a fluorescent signal transducer due to their

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tremendous photophysical properties like as extended absorption and emission wavelengths,

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high fluorescence quantum yield and large absorption coefficient. In fact, a longer

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wavelength emission (~550 nm) was often preferred to serve as a sensing signal to avoid the 5

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background fluorescence influence (below 500 nm) [36,37]. Recently, several rhodamine-

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based probes as fluorescent chemosensors for metal ions have been developed [38–45]. Herein, we report the synthesis of two novel rhodamine derivatives L1 and L2, which

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show a reversible, selective and sensitive fluorescence enhancement response to Cr(III) in

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alcoholic media.

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2. Experimental

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2.1. Reagents and apparatus

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Rhodamine B, metal salts and other commercially obtainable chemicals were

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purchased from Merck and Aldrich and used without further purification. The melting point

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was measured on a SRS OptiMelt Automated melting point system. The IR spectra were

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recorded on a PerkinElmer FT-IR spectrometer (USA) in the range 4000–400 cm−1 with KBr.

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The NMR spectra were measured by using Bruker 500 MHz (USA), TMS as an internal

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standard, CDCl3, DMSO-d6 and CD3OD are taken as solvents. The mass spectra were

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recorded on a Bruker-micrOTOF II (USA). The UV-vis absorption spectra were obtained on

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a Shimadzu UV-2450 spectrophotometer (Japan) and the Fluorescent spectra were recorded

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by using Shimadzu RF-5301PC spectrofluorophotometer (Japan). Differential Pulse

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Voltammetric experiments were performed using a CHI760E electrochemical workstation

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(USA) with a conventional three-electrode configuration consisting of a glassy carbon

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working electrode, a platinum wire counter electrode, and an aqueous Ag/AgNO3 reference

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electrode. The pH was measured with a Eutech CyberScan pH 510 (Singapore).

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2.2. Synthesis and characterisation

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The synthetic route for Chemosensors (L1 and L2) was outlined in Scheme 1.

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Chemosensors were prepared by following the literature method [46] and the structures were

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characterised by FT-IR, 1H NMR, 13C NMR and ESI-MS spectra (Fig. 1−3).

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Compound 1: Ethylenediamine (2.5 ml) was added drop wise to the ethanolic solution of

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rhodamine B (2.0 g) with vigorous stirring at room temperature. On completion of addition,

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the stirred solution was allowed to reflux about 6–8 h. The color of mixture changed from

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dark pink to light orange. Then the mixture was cooled and solvent was removed under

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reduced pressure. 1 M HCl (about 50 mL) was added to the reaction mixture to produce a

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clear red solution. Later than, 1 M NaOH in water was added gradually with constant stirring

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until the pH of the solution reached 9–10. The resulting precipitate was filtered and washed

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4–5 times with 15 mL water. After drying under reduced pressure, the reaction yielded 1.8 g

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1 (90%) as pink solid. Mp: 215–217 °C; FT-IR (KBr), ν, cm−1: 1620 (C=O), 1385, 1121

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(C−N), 1224, 1021 (C−O); 1H NMR (CDCl3, 500 MHz), δ (ppm): 1.16 (12H, t, J = 7.0 Hz),

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2.42 (2H, t, J = 6.0 Hz), 3.19 (2H, t, J = 6.0 Hz), 3.33 (8H, q, J = 7.0 Hz), 6.27 (2H, d, J = 8.5

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Hz), 6.37 (2H, s), 6.43 (2H, d, J = 8.5 Hz), 7.09 (1H, s), 7.44 (2H, s), 7.90 (1H, s); 13C NMR

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(CDCl3, 500 MHz), δ (ppm): 12.6, 40.8, 43.8, 44.3, 65.0, 97.7, 105.7, 108.2, 122.7, 123.8,

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128.0, 128.7, 131.2, 132.4, 148.8, 153.3, 153.5, 168.6. ESI-MS m/z: Calcd for C30H36N4O2

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(M+H)+: 485.2917, found: 485.2762.

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Compound L1 and L2:

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Compound 1 (0.24 g, 0.5 mmol) and aldehyde (0.5 mmol) were dissolved in 20 mL absolute

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ethanol. The reaction mixture was stirred for 6 h at room temperature. Obtained solid was

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filtered and washed 3 times with 10 mL ethanol. After drying under reduced pressure, the

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reaction afforded 0.26 g L1 (82%) as yellow solid and 0.22 g L2 (73%) as white solid,

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respectively.

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Compound L1: Mp: 167–169 °C; FT-IR (KBr), ν, cm−1: 1620 (C=O), 1365, 1121 (C−N),

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1227, 1012 (C−O); 1H NMR (CDCl3, 500 MHz), δ (ppm): 1.16 (12H, t, J = 7.0 Hz), 3.31

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(8H, q, J = 6.5 Hz), 3.39–3.45 (4H, m), 6.24 (2H, d, J = 8.5 Hz), 6.41–6.44 (4H, m), 6.84

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(1H, d, J = 9.0 Hz), 7.10 (1H, s), 7.19 (1H, t, J = 7.5 Hz), 7.39 (1H, t, J = 7.5 Hz), 7.45 (2H,

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s), 7.55 (1H, d, J = 7.5 Hz), 7.61 (1H, d, J = 9.0 Hz), 7.78 (1H, d, J = 8.0 Hz), 7.93 (1H, s),

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8.50 (1H, s), 13.99 (1H, s);

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65.1, 97.8, 105.1, 106.8, 108.2, 118.0, 122.6, 122.9, 123.9, 124.9, 126.2, 127.8, 128.2, 128.7,

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129.1, 130.9, 132.6, 133.9, 137.1, 148.9, 153.4, 158.5, 168.4, 176.3. ESI-MS m/z: Calcd for

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C41H42N4O3 (M+H)+: 639.3335, found: 639.3147.

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Compound L2: Mp: 205–207 °C; FT-IR (KBr), ν, cm−1: 1626 (C=O), 1397, 1115 (C−N),

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1218 (C−O); 1H NMR (CDCl3, 500 MHz), δ (ppm): 1.16 (12H, t, J = 6.0 Hz), 3.21 (2H, t, J =

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6.5 Hz), 3.32 (8H, d, J = 6.5 Hz), 3.42 (2H, t, J = 6.0 Hz), 6.25–6.31 (3H, m), 6.39–6.42 (5H,

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m), 6.82 (1H, d, J = 8.0 Hz), 7.08 (1H, d, J = 5.0 Hz), 7.43 (2H, s), 7.75 (1H, s), 7.90 (1H, s);

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108.2, 111.7, 123.0, 123.8, 128.2, 128.8, 130.8, 132.6, 133.4, 148.9, 153.3, 153.5, 162.0,

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164.8, 167.0, 168.6. ESI-MS m/z: Calcd for C37H40N4O4 (M+Na)+: 627.2947, found:

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627.2942.

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C NMR (CDCl3, 500 MHz), δ (ppm): 12.6, 40.9, 44.3, 50.6,

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C NMR (CDCl3, 500 MHz), δ (ppm): 12.6, 40.9, 44.3, 54.5, 65.3, 97.8, 103.8, 105.1, 107.1,

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2.3. UV-vis and Fluorescent studies Stock solutions of 1 × 10−3 M various metal ions and receptor were prepared in

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methanol and MeOH−DMSO (99:1 v/v), respectively. The solutions were then diluted to 1 ×

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10−4 M using same solvents. All measurements of UV-vis absorption and fluorescence

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emission spectra were carried out in 1.0 cm path length quartz cuvettes at room temperature.

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Absorption and emission spectra of the chemosensor in the presence of various metal ions

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were measured in the concentration of 50 µM. Stoichiometry, binding constant of sensing 8

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probe–Cr3+ complex, limit of detection of Cr3+ and quantum yield were calculated by using

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spectrofluorophotometer. For all the fluorescence emission measurements, excitation

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wavelength was 520 nm, and both the excitation and emission slit widths were 1.5 and 3 nm,

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respectively.

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3. Results and discussion

The binding ability and mode of chemosensors toward Cr3+ were investigated through absorption, emission, electrochemical, ESI-MS, DFT calculation and 1H NMR experiments.

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3.1. Absorption spectroscopic studies

The binding ability of probe (50 µM) against different metal ions (50 µM) such as

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Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Gd3+, Hg2+, Mn2+, Nd3+, Ni2+,Pb2+ and Zn2+ were carried

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out by UV-vis absorption studies. As observed, the UV-vis spectra of L1 and L2 exhibited an

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absorption band in the 350–450 nm region, on addition of Cr3+ ion (1:1, v/v) lead to form of a

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strong absorption transition at ~555 nm with a shoulder at ~518 nm (Fig. 4). The noticeable

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naked eye recognition of the pink color development in these probes (Fig. 4, inset) upon

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Cr(III) addition implies a metal-induced delactonization of rhodamine, while the rest of the

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metal ions induced a insignificant absorption change even when added in excess. On

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complexation, initial spirolactam form of probe is converted into its ring opened amide

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conformation [47].

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The plot of absorbance at 555 nm of L1 and L2 as a function of mole fraction of Cr3+

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ions (Jobs plot) exposes that these probes bind to the chromium metal ion in 1:1

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stoichiometry (Fig. 5).

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3.2. Fluorescence emission studies 9

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The fluorescence spectral pattern (Fig. 6) of L1 and L2 when excited at 520 nm in the

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presence of different metal ions exhibited that their non-fluorescent behaviour becomes

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highly fluorescent upon metal addition (OFF-ON). Under a UV lamp, showed a fabulous

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color change from colorless to brick red in the solution of probes upon the addition of Cr3+,

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which could simply be identified by the naked-eye (Fig. 6, inset). This implies a

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delactonization process, of the non-fluorescent spirocyclic form to its highly fluorescent ring

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opened form of rhodamine which is induced by metal ion coordination. The degree of

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chelation-enhanced fluorescence effects depend on the character of the ligands and

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interacting metal ions. The binding ability depends on size, charge and electron configuration

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of the metal ion and ligand. Those characters of metal ion and ligand are very suitable for

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each other to form metal complex. Amongst all the metal ions examined, these probes

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displayed high fluorescence enhancement at λem = ~575 nm in the presence of Cr(III) ion.

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Probe L1 exhibited maximum Cr(III)-induced fluorescence enhancement (114-fold) than L2

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(38-fold), showing its higher affinity towards Cr(III) with a good response time (<5 seconds)

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in comparison to other probe (L2).

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The emission spectral pattern of L1 and L2 (50 µM) upon addition of increasing

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concentration (0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 µM) of Cr3+ ion, a

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new emission band peaked at ~575 nm appeared with increasing intensity (Fig. 7). The

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complex stability constants (K) through Benesi-Hildebrand method for Cr(III) with L1 and

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L2 were found to be 2.7×104 M−1 and 4.5×103 M−1, respectively (Fig. 7, inset). The

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observable brick red color development in these probes due to a highly delocalized π-

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conjugated system of probes was formed. The detection limit of Cr(III) was calculated based

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on the fluorescence titration profile as 4.9×10−8 M (for L1) and 2.4×10−7 M (for L2) based on

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S/N = 3 (Fig. 7, inset).

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Furthermore, to determine the stoichiometry of probe–Cr3+ complex, continuous

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variation (Job’s) method was conducted (Fig. 8). As supposed, the results show the formation

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of a 1:1 stoichiometry complex between Cr3+ and probe, and stoichiometric ratio was further

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confirmed by ESI-MS analysis (Fig. 9). Observed mass peak at m/z 742.2372 and 708.2255

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corresponding to [L1 + Cr + Cl + H2O − H]+ and [L2 + Cr + Cl + H2O − H]+ respectively,

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which are solid evidence for the formation of a 1:1 complex.

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In addition, to verify the selectivity of these ligands towards Cr(III) ions over various

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competitive metal ions. The emission intensity changes of L1 and L2 (50 µM) upon addition

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of other metal ions (50 µM) and Cr(III) along with other metal ions were evaluated (Fig. 10).

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The results exposed that Cr(III)-induced fluorescence response was unaffected in the

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presence of other interfering ions used. This experiment establishes the significant feature of

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high selectivity of these probes towards Cr(III) over other competitive metal ions.

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In acidic media, the spirolactam ring of the rhodamine and its derivatives is open and

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then shows the absorbance and fluorescence characteristics of rhodamine. The absorbance

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and fluorescence responses of probes in the presence of Cr(III) in different pH value were

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estimated (Fig. 11). The absorbance spectra of probe–Cr(III) is gradually increased from pH

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2 to 4 and reached a λmax at pH 4. From pH 4 to 6, the absorbance maxima moved downward.

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A rapid fluorescence enhancement accompanied by a red shift was observed with pH

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variation from 2 to 4. The fluorescence quenching accompanied by a blue shift was started

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while changing in the pH from 4 to 6. Absorbance (at 555 nm) and fluorescence emission (at

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575 nm) of probe–Cr(III) disappeared in basic conditions (≥7). The same spectral changes

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were observed for probe (L1 and L2) alone in various pH conditions. The chemosensors (L1

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and L2) in the presence of Cr(III) exhibited a dramatic color changes in the different pH

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media, which could simply be identified by the naked-eye (Fig. 11, inset). Absorbance and

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Emission enhancement factor, and corresponding quantum yields of L1 and L2 with pH

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variation in the absence and presence of Cr(III) are collected in Table 1. To examine the reversibility of complexation of probe towards Cr(III) ion, EDTA

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titration experiments were conducted. Upon addition of EDTA to the solution containing

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probe (L1 and L2) and Cr(III) weaken the fluorescence intensity significantly, whereas

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readdition of excess Cr(III) ion could recover the fluorescence emission signal (Fig. 12). As

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results show, it could provide as experimental evidence to support the reversibility of spiro

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ring-opening and closing mechanism. The proposed binding mechanism of probe (L1 and

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L2) with Cr(III) in the presence and absence of EDTA was shown in the Scheme 2.

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Both UV-vis and fluorescence emission results indicate that probes show a good selectivity and sensitivity toward Cr(III) over other metal ions.

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3.3. Electrochemical measurements

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As shown in Figure 13, the band gap energy related wavelength for probes is obtained

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from the cross point of absorption onset line and corrected base line [48]. The corresponding

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wavelengths are 444 and 424 nm, and are equal to 2.79 and 2.92 eV energy band gap for L1

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and L2, respectively. The corresponding wavelength to the band gap energy for probes with

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Cr(III) can be determined from the cross point of absorption and emission onset lines (Fig.

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14). The corresponding wavelengths are 565 and 564 nm which are equal to 2.19 and 2.20 eV

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energy band gap for L1+Cr(III) and L2+Cr(III), respectively.

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The current-voltage curve for probes (L1 and L2) in the absence and presence of

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Cr(III) regarding to Differential Pulse Voltammetric experiments are shown in Figure 15.

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Based on results, L1 and L2 alone show Eox= 0.552 and 0.556 V which are equal to EHOMO=

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−5.35 and −5.36 eV, respectively. The probes in the presence of Cr(III) ions show Eox= 0.572

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and 0.596 V which are equal to EHOMO= −5.37 (for L1+Cr3+) and −5.39 eV (for L2+Cr3+). By 12

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addition of Cr(III) ion changes are occurred in the oxidation potentials of probes, due to

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decrease in electron releasing nature of probe-Cr3+ complexes. LUMO energy levels (for L1,

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L2, L1+Cr3+ and L2+Cr3+ are −2.56, −2.44, −3.18 and −3.19 eV, respectively) were

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estimated from HOMO and band gap energies.

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This experiment proves that, increase in oxidation potential and decrease in band gap

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due to strong interactions between probes (L1 and L2) and chromium ion. Figure 16 shows

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the energy diagram with HOMO/LUMO levels of probes alone and in the presence of Cr(III).

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3.4. Density functional theory (DFT) calculations

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To better understand the nature of the coordination of Cr3+ with L1 and L2, energy-

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optimized structures of L1, L2 and its corresponding Cr3+ complexes (Fig. 17) were obtained

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on density functional theory (DFT) calculations at the B3LYP level using 6-31G** basis set

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for simple ionophores (L1 and L2) and LANL2DZ basis set for metal complexes using the

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Gaussian 09 program [49]. The spatial distributions and orbital energies of the highest

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occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)

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of L1, L2 and its corresponding Cr3+ complexes were also generated using this calculations

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(Fig. 18). As results indicated, the HOMO is distributed over the xanthene moiety, while

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LUMO is spread on the naphthol part (in L1) and phenylene part (in L2). As result shown,

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the spirocyclic C–N bond breaks to facilitate the binding of Cr3+ ion with the carbonyl

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oxygen atom of ligand. The sensors have also the capability to bind metal ion through

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phenolic –OH. The π electrons of HOMO orbitals of L1–Cr3+ and L2–Cr3+ are mainly located

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on the naphthol and phenylene moiety, respectively and the LUMO is mostly spread around

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the metal ion. The energy gaps between the HOMO and LUMO of the probes (L1 and L2)

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and its corresponding Cr3+ complexes were calculated as 3.906 eV, 4.433 eV, 2.727 eV and

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2.719 eV, respectively. The results exhibited that the binding of Cr3+ to probe lowered the

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HOMO−LUMO energy gap and stabilized the system. Thus, they show a favourable

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complexation according to proposed coordination.

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3.5. 1H NMR titration

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Furthermore, to better understand the interaction between probe L1 and Cr3+, proton

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NMR titration experiment was performed in the presence of various amounts of Cr3+ in a

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DMSO-d6+CD3OD solvent (Fig. 19). Upon complexation with Cr3+ ion, the signals of imine

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proton (Hg) and an aryl proton (Ha) of 2-naphthol downfield shifted from 8.68 ppm to 8.79

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ppm and 6.67 ppm to 6.82 ppm, respectively. The signals of protons Hb and Hc of 2-naphthol

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were combined with each other and gave a single peak at 7.8 ppm. Similarly the signals of

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aryl-protons Hi, Hj of rhodamine moiety and He of 2-naphthol were come together then gave

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a typical complex signal. Upon addition of Cr3+ ions, a combine signal of aryl-protons (Hm,

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Hn) of rhodamine moiety was splitted into two signals of Hm and Hn. The distance between

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two signals of Hd and Hk of 2-naphthol and rhodamine moiety, respectively, is also varied

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upon addition of Cr3+. Other signals of aryl-protons (Hf, Hh) of 2-naphthol and rhodamine

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moiety also downfield shifted because the strong coordination between L1 and Cr3+ ion.

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To investigate the practical application of chemosensor (L1 and L2), polymeric thin

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films were prepared [50]. Polyvinyl chloride (PVC) (100 mg), Bis(2-ethylhexyl)sebacate (as

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plasticizer) (200 mg) and probe were dissolved in THF (5 ml). The homogeneous mixture

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obtained after completion of dissolution of all ingredients was concentrated by evaporation of

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THF at room temperature. This homogeneous mixture was poured onto a clean glass surface.

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The solvent was allowed to evaporate and obtained non-fluorescent polymeric membrane

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sensor was used for Cr3+ detection. A solution containing Cr3+ in methanol (1 mM) was

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sprayed onto the film, on the solvent evaporation a strong fluorescent image appeared on the

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Cr3+ exposed regions (Fig. 20). 14

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4. Conclusion The newly synthesized rhodamine based fluoroionophores L1 and L2 exhibit a good

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selective and sensitive toward Cr3+ ion over other tested metal ions in MeOH−DMSO (99:1

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v/v). The binding ability and mode of chemosensors toward Cr3+ were investigated through

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absorption, emission, electrochemical, ESI-MS and 1H NMR experiments. The reported

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fluoroionophores exhibited a reversible absorption and fluorescence enhancement response to

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Cr(III) via a 1:1 binding mode at neutral pH. A polymeric thin film can be obtained by

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doping PVC with chemosensor L1 and L2. Such a thin film can be used as a sensor to detect

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Cr3+ with high selectivity.

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Acknowledgements

Naveen is grateful to the Ministry of Human resource Development (MHRD), New

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Delhi, India for financial support and also thankful to the departmental Instrumentation lab

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(IITR) for providing the spectrophotometer, spectrofluorophotometer, cyclic voltammetry

323

and other facilities.

325 326 327 328

te

Ac ce p

324

d

320

329 330 331 332 15

Page 15 of 49

References

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335

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assessment of chromium in the environment, Crit. Rev. Environ. Sci. Technol. 29 (1999) 1.

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[3] A.K. Singh, V.K. Gupta, B.H. Gupta, Chromium(III) selective membrane sensors based

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on Schiff bases as chelating ionophores, Anal. Chim. Acta 585 (2007) 171.

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[4] M. Sarkar, S. Banthia, A. Samanta, A highly selective ‘off–on’ fluorescence chemosensor

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for Cr(III), Tetrahedron Lett. 47 (2006) 7575.

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[5]. M.H. Mashhadizadeh, M. Pesteh, M. Talakesh, I. Sheikhshoaie, M.M. Ardakani, M.A.

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coupled plasma optical emission spectrometry, Spectrochim. Acta B 57 (2002) 1939.

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[11]. A. Mohadesi, M.A. Taher, Voltammetric determination of Cu(II) in natural waters and

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preconcentration and detection of neuropeptides at patterned graphene-modified electrodes in

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electrochemical monitoring of adenosine triphosphate in the picomolar to micromolar range

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using graphene-modified electrodes, Anal. Chem. 85 (2013) 8158.

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[15] B.J. Sanghavi, S.M. Mobin, P. Mathur, G.K. Lahiri, A.K. Srivastava, Biomimetic sensor

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for certain catecholamines employing copper(II) complex and silver nanoparticle modified

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glassy carbon paste electrode, Biosens. Bioelectron. 39 (2013) 124.

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[16] B.J. Sanghavi, A.K. Srivastava, Simultaneous voltammetric determination of

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acetaminophen, aspirin and caffeine using an in situ surfactant-modified multiwalled carbon

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nanotube paste electrode, Electrochim. Acta 55 (2010) 8638.

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determination of venlafaxine and desvenlafaxine employing Nafion–carbon nanotube

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composite glassy carbon electrode, Electrochim. Acta 56 (2011) 4188.

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[18] V.K. Gupta, A.K. Jain, S.K. Shoora, Multiwall carbon nanotube modified glassy carbon

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electrode as voltammetric sensor for the simultaneous determination of ascorbic acid and

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caffeine, Electrochim. Acta 93 (2013) 248.

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[19] V.K. Gupta, A.K. Singh, L.K. Kumawat, A novel gadolinium ion-selective membrane

382

electrode based on 2-(4-phenyl-1, 3-thiazol-2-yliminomethyl) phenol, Electrochim. Acta 95

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(2013) 132.

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[20] M. Zhao, L. Ma, M. Zhang, W. Cao, L. Yang, L.J. Ma, Glutamine-containing "turn-on"

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fluorescence sensor for the highly sensitive and selective detection of chromium (III) ion in

386

water, Spectrochim. Acta A 116 (2013) 460.

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[21] Z. Zhou, M. Yu, H. Yang, K. Huang, F. Li, T. Yi, C. Huang, FRET-based sensor for

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imaging chromium(III) in living cells, Chem. Commun. (2008) 3387.

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[22] Y.J. Jang, Y.H. Yeon, H.Y. Yang, J.Y. Noh, I.H. Hwang, C. Kim, A colorimetric and

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fluorescent chemosensor for selective detection of Cr3+ and Al3+, Inorg. Chem. Commun. 33

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(2013) 48.

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[23] H.W. Wang, Y.Q. Feng, C. Chen, J.Q. Xue, Two novel fluorescent calix[4]arene

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derivatives with benzoazole units in 1,3-alternate conformation for selective recognition to

394

Fe3+ and Cr3+, Chin. Chem. Lett. 20 (2009) 1271.

395

[24] X. Wang, Y. Wei, S. Wang, L. Chen, Red-to-blue colorimetric detection of chromium

396

via Cr (III)-citrate chelating based on Tween 20 stabilized gold nanoparticles, Colloids Surf.

397

A 472 (2015) 57.

398

[25] V.K. Gupta, A.K. Singh, M.R. Ganjali, P. Norouzi, F. Faridbod, N. Mergu, Comparative

399

study of colorimetric sensors based on newly synthesized schiff bases, Sens. Actuators B 182

400

(2013) 642.

401

[26] V.K. Gupta, A.K. Singh, N. Mergu, Antipyrine based schiff bases as turn-on fluorescent

402

sensors for Al(III) ion, Electrochim. Acta 117 (2014) 405.

403

[27] V.K. Gupta, A.K. Singh, L.K. Kumawat, Thiazole schiff base turn-on fluorescent

404

chemosensor for Al3+ ion, Sens. Actuators B 195 (2014) 98.

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d

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18

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[28] V.K. Gupta, N. Mergu, A.K. Singh, Fluorescent chemosensors for Zn2+ ions based on

406

flavonol derivatives, Sens. Actuators B 202 (2014) 674.

407

[29] V.K. Gupta, A.K. Singh, L.K. Kumawat, A turn-on fluorescent chemosensor for Zn2+

408

ions based on antipyrine schiff base, Sens. Actuators B 204 (2014) 507.

409

[30] V.K. Gupta, N. Mergu, L.K. Kumawat, A.K. Singh, Selective naked-eye detection of

410

magnesium (II) ions using a coumarin-derived fluorescent probe, Sens. Actuators B 207

411

(2015) 216.

412

[31] V.K. Gupta, S.K. Shoora, L.K. Kumawat, A.K. Jain, A highly selective colorimetric and

413

turn-on fluorescent chemosensor based on 1-(2-pyridylazo)-2-naphthol for the detection of

414

aluminium(III) ions, Sens. Actuators B 209 (2015) 15.

415

[32] N. Mergu, V.K. Gupta, A novel colorimetric detection probe for copper(II) ions based

416

on a schiff base, Sens. Actuators B 210 (2015) 408.

417

[33] L.K. Kumawat, N. Mergu, A.K. Singh, V.K. Gupta, A novel optical sensor for copper

418

ions based on phthalocyanine tetrasulfonic acid, Sens. Actuators B 212 (2015) 389.

419

[34] H.N. Kim, M.H. Lee, H.J. Kim, J.S. Kim, J. Yoon, A new trend in rhodamine-based

420

chemosensors: application of spirolactam ring-opening to sensing ions, Chem. Soc. Rev. 37

421

(2008) 1465.

422

[35]

423

of Rhodamine derivatives as fluorescent probes, Chem. Soc. Rev. 38 (2009) 2410.

424

[36] G. Grynkiewicz, M. Poenie, R.Y. Tsien, A new generation of Ca2+ indicators with

425

greatly improved fluorescence properties, J. Biol. Chem. 260 (1985) 3440.

426

[37] A. Minta, R.Y. Tsien, Fluorescent indicators for cytosolic sodium, J. Biol. Chem. 264

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(1989) 19449.

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te

d

M

an

us

cr

ip t

405

M.

Beija,

C.A.M.

Afonso,

J.M.G.

Martinho,

Synthesis

and

applications

19

Page 19 of 49

[38] C. Yu, J. Zhang, R. Wang, L. Chen, Highly sensitive and selective colorimetric and off-

429

on fluorescent probe for Cu2+ based on rhodamine derivative, Org. Biomol. Chem. 8 (2010)

430

5277.

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[39] C. Kaewtong, B. Wanno, Y. Uppa, N. Morakot, B. Pulpoka, T. Tuntulani, Facile

432

synthesis of rhodamine-based highly sensitive and fast responsive colorimetric and off-on

433

fluorescent reversible chemosensors for Hg2+: preparation of fluorescent thin film sensor,

434

Dalton Trans. 40 (2011) 12578.

435

[40] A. Sahana, A. Banerjee, S. Lohar, A. Banik, S.K. Mukhopadhyay, D.A. Safin, M.G.

436

Babashkina, M. Bolte, Y. Garcia, D. Das, FRET based tri-color emissive rhodamine-pyrene

437

conjugate as an Al3+ selective colorimetric and fluorescence sensor for living cell imaging,

438

Dalton Trans. 42 (2013) 13311.

439

[41] M.H. Lee, J.S. Wu, J.W. Lee, J.H. Jung, J.S. Kim, Highly sensitive and selective

440

chemosensor for Hg2+ based on the rhodamine fluorophore, Org. Lett. 9 (2007) 2501.

441

[42] J. Du, J. Fan, X. Peng, P. Sun, J. Wang, H. Li, S. Sun, A new fluorescent

442

chemodosimeter for Hg2+: selectivity, sensitivity, resistance to Cys and GSH, Org. Lett. 12

443

(2010) 476.

444

[43] C. Yu, L. Chen, J. Zhang, J. Li, P. Liu, W. Wang, B. Yan, “Off-On” based fluorescent

445

chemosensor for Cu2+ in aqueous media and living cells, Talanta 85 (2011) 1627.

446

[44] C. Yu, J. Zhang, J. Li, P. Liu, P. Wei, L. Chen, Fluorescent probe for copper(II) ion

447

based on a rhodamine spirolactame derivative, and its application to fluorescent imaging in

448

living cells, Microchim. Acta 174 (2011) 247.

449

[45] N. Mergu, A.K. Singh, V.K. Gupta, Highly sensitive and selective colorimetric and off-

450

on fluorescent reversible chemosensors for Al3+ based on the rhodamine fluorophore, Sensors

451

15 (2015) 9097.

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d

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Page 20 of 49

[46] Y. Xiang, A. Tong, P. Jin, Y. Ju, New fluorescent rhodamine hydrazone chemosensor

453

for Cu(II) with high selectivity and sensitivity, Org. Lett. 8 (2006) 2863.

454

[47] B. Bag, A. Pal, Rhodamine-based probes for metal ion-induced chromo-/fluorogenic

455

dual signalling and their selectivity towards Hg(II) ion, Org. Biomol. Chem. 9 (2011) 4467.

456

[48] A. Shafiee, M.M. Salleh, M. Yahaya, Determination of HOMO and LUMO of [6,6]-

457

phenyl C61-butyric acid 3-ethylthiophene ester and poly (3-octyl-thiophene-2, 5-diyl)

458

through voltametry characterization, Sains Malays. 40 (2011) 173.

459

[49] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G.

460

Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P.

461

Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara,

462

K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,

463

T. Vreven, J.A. Montgomery, Jr. J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers,

464

K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C.

465

Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B.

466

Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J.

467

Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski,

468

G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B.

469

Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision A.02, Gaussian Inc.,

470

Wallingford, CT, 2009.

471

[50] V.K. Gupta, A.K. Singh, N. Mergu, A new beryllium ion-selective membrane electrode

472

based on dibenzo(perhydrotriazino)aza-14-crown-4 ether, Anal. Chim. Acta 749 (2012) 44.

Ac ce p

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473 474 475 476 21

Page 21 of 49

Figure captions:

478

Scheme 1. Synthetic Pathways of L1 and L2.

479

Fig. 1. 1H NMR Spectrum of 1, L1 and L2.

480

Fig. 2. 13C NMR Spectrum of 1, L1 and L2.

481

Fig. 3. HRMS Spectrum of 1, L1 and L2.

482

Fig. 4. Absorbance spectra of L1 (a) and L2 (b) (50 µM) in presence of various metal ions

483

(50 µM) in MeOH−DMSO (99:1 v/v). Inset: Color change of probe in the presence of Cr3+.

484

Fig. 5. Job’s plot for L1 and L2 with Cr3+, absorbance intensity at 555 nm was plotted as a

485

function of the molar ratio.

486

Fig. 6. Fluorescence spectra (λex = 520 nm) of L1 (a) and L2 (b) (50 µM) in presence of

487

various metal ions (50 µM) in MeOH−DMSO (99:1 v/v). Inset: Color change of probe in the

488

presence of Cr3+.

489

Fig. 7. The variation in fluorescence emission spectra of L1 (a) and L2 (b) in the presence of

490

increasing concentrations of Cr3+ (0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100

491

µM). Inset: Linear regression plot of fluorescence intensity change 1/(I-I0) as a function of

492

concentration 1/[Cr3+] (top), fluorescence enhancement change as a function of concentration

493

of Cr(III) added (bottom).

494

Fig. 8. Job’s plot for L1 and L2 with Cr3+, fluorescence intensity at 575 nm was plotted as a

495

function of the molar ratio.

496

Fig. 9. ESI-MS spectrum of L1 (a) and L2 (b) upon addition of CrCl3.6H2O (1.0 equiv.) in

497

MeOH.

498

Fig. 10. Competitive selectivity of probes L1 (a) and L2 (b) toward various metal ions (1.0

499

equiv.) in the absence (black bars) and presence (red bars) of Cr3+ (1.0 equiv.) with an

500

excitation of 520 nm.

Ac ce p

te

d

M

an

us

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ip t

477

22

Page 22 of 49

Fig. 11. UV-vis absorbance spectral changes of L1 (a) and L2 (b), Fluorescence emission

502

intensities (both excitation and emission slit widths were 1.5 nm) of L1 (c) and L2 (d) with

503

Cr3+ as a function of pH. Inset: Color changes of L1+Cr3+ and L2+Cr3+ in different pH media

504

under a normal (a, b) and fluorescent (c, d) light (top), absorbance (a and b, at 555 nm) and

505

emission (c and d, at 575 nm) intensities of L1 and L2 in the presence of Cr3+ with pH

506

variation (bottom).

507

Fig. 12. The variation in fluorescence emission spectra of L1+Cr3+ (a) and L2+Cr3+ (b) upon

508

addition of EDTA (0, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 µM). Inset: Color

509

changes of probe+Cr3+ upon addition of EDTA (2 equiv.) (Top left), fluorescence spectral

510

changes at 575 nm as a function of the amount of EDTA (right) and recovery of the

511

molecular fluorescence at 575 nm of the sensor (50 µM) after addition of Cr3+ (1 equiv.) after

512

each addition of 2 equiv. of EDTA (Bottom left).

513

Scheme 2. Proposed binding mechanism of Cr(III) with probes in the presence and absence

514

of EDTA.

515

Fig. 13. Absorption spectra and optical band gaps of probes L1 and L2.

516

Fig. 14. UV-vis absorption and fluorescence emission spectra of ligands and the

517

corresponding Cr3+ addition products in MeOH−DMSO (99:1 v/v).

518

Fig. 15. Differential pulse voltammograms recorded for L1 (a) and L2 (b), and the

519

corresponding Cr3+ addition products in MeOH−DMSO (99:1 v/v).

520

Fig. 16. Energy level diagram of the probes and the corresponding Cr3+ addition products.

521

Fig. 17. Optimized structures of L1 and L2 and the corresponding Cr3+ addition products.

522

Fig. 18. HOMO and LUMO orbitals of probes (L1 and L2) and its corresponding Cr3+

523

complexes.

524

Fig. 19. 1H NMR titration of L1 with Cr3+ in DMSO-d6+CD3OD.

Ac ce p

te

d

M

an

us

cr

ip t

501

23

Page 23 of 49

525

Fig. 20. Fluorescence image of PVC polymeric thin film doped with ligand. The polymeric

526

film on the glass slide was irradiated with a UV lamp.

527

ip t

528 529

cr

530 531

us

532

an

533 534

M

535 536

540 541 542 543 544 545

te

539

Ac ce p

538

d

537

546 547 548 549 24

Page 24 of 49

O H2N

COOH

O

NH2

N

N

N

O

Cl

N

1

ip t

N

NH2

CHO

OHC

OH

cr

HO

us

OH

O

O

N

N

553 554 555 556 557 558 559

M

L1

O

N

L2

Scheme 1. Synthetic Pathways of L1 and L2.

d

552

N

HO

te

551

N

OH

Ac ce p

550

O

an

HO N

N

N

560 561 562 563 25

Page 25 of 49

ip t cr M

an

us

564

568 569 570 571 572 573

te

567

Fig. 1. 1H NMR Spectrum of 1, L1 and L2.

Ac ce p

566

d

565

574 575 576 577 26

Page 26 of 49

ip t cr M

an

us

578

579

582 583 584 585 586 587

d

te

581

Fig. 2. 13C NMR Spectrum of 1, L1 and L2.

Ac ce p

580

588 589 590 591 27

Page 27 of 49

ip t us

cr 593 594 595 596 597

Ac ce p

te

d

M

an

592

Fig. 3. HRMS Spectrum of 1, L1 and L2.

598 599 600 601 28

Page 28 of 49

0.80

b

a 0.64

0.3 2+

L1, Cd , Co2+, Fe2+, 3+

3+

3+

2+

Fe , Gd 0.2

2+

2+

2+

2+

Hg , Mn ,

3+

Cr

Nd , Ni , Pb , Zn

Absorbance

Absorbance

0.4

0.1

2+

2+

Cu2+, Ni , Zn

0.48

2+

3+

L2, Cd , Co2+, Fe2+, Fe , 3+

2+

3+

0.00

602

400

450

500

550

340

600

374

408

442

476

510

cr

350

2+

Cr

Cu

0.0

3+

0.32

0.16

2+

2+

Gd , Hg , Mn , Nd , Pb

ip t

0.5

544

578

Wavelength (nm)

Wavelength (nm)

Fig. 4. Absorbance spectra of L1 (a) and L2 (b) (50 µM) in presence of various metal ions

604

(50 µM) in MeOH−DMSO (99:1 v/v). Inset: Color change of probe in the presence of Cr3+.

us

603

an

605 606

M

607

611 612 613 614 615 616

te

610

Ac ce p

609

d

608

617 618 619 620 29

Page 29 of 49

0.4

L1+Cr L2+Cr

3+ 3+

0.2

ip t

Absorbance

0.3

0.0

0.2

0.4

621

0.6

Mole fraction of Cr

0.8

3+

1.0

us

0.0

cr

0.1

Fig. 5. Job’s plot for L1 and L2 with Cr3+, absorbance intensity at 555 nm was plotted as a

623

function of the molar ratio.

an

622

M

624 625

629 630 631 632 633 634

te

628

Ac ce p

627

d

626

635 636 637

30

Page 30 of 49

b

a 240

550

440 3+

Cr 330

220

2+

L1, Cd , Co2+, Cu2+, 3+

3+

200

160 3+

Cr 120 2+

L2, Cd , Co2+, Fe2+,

80

2+

3+

2+

2+ 2+

3+

2+

3+

2+

2+

Cu2+

0

0 559

638

572

585

598

611

624

637

650

533

546

559

572

585

598

611

624

637

650

cr

546

2+

Nd , Ni , Pb , Zn

40

2+

Mn , Nd , Ni , Pb , Zn

533

2+

Fe , Gd , Hg , Mn ,

Fe2+, Fe , Gd , Hg , 110

3+

ip t

Fluorescence Intensity

Fluorescence Intensity

660

Wavelength (nm)

Wavelength (nm)

Fig. 6. Fluorescence spectra (λex = 520 nm) of L1 (a) and L2 (b) (50 µM) in presence of

640

various metal ions (50 µM) in MeOH−DMSO (99:1 v/v). Inset: Color change of probe in the

641

presence of Cr3+.

an

us

639

642

M

643

647 648 649 650 651 652

te

646

Ac ce p

645

d

644

653 654 655 656 31

Page 31 of 49

a

0.025

1.0

b

400

y = 0.1293x - 0.0008 R² = 0.9888 K = 2.7×104 M-1

0.020

0.8

0.005

0.000

600

0.00

0.05

0.10 3+

0.15

0.20

1/[Cr ]/µM 1000

400

100 µM 3+

Cr 200

y = 9.594x + 44.705 R² = 0.9459 LOD = 4.9×10-8 M

800

0.0 µM

600

1/I-Io

300

0.0

0.00

100 µM 3+

Cr 100

0.0 µM

200

40

0

80

575

600

625

650

675

700

100

190

95

0

550

750

20

40

60

80

100

[Cr3+]/µM

0 725

0.20

575

600

625

650

675

700

725

750

cr

657

60

[Cr3+]/µM

0.15

y = 4.3545x - 42.334 R² = 0.9532 LOD = 2.4×10-7 M

285

0

20

0.10 3+

380

400

0

0.05

1/[Cr ]/µM

200

0

550

0.4

0.2

Fluorescence Intensity

Fluorescence Intensity

1/I-Io

0.010

Fluorescence Intensity

Fluorescence Intensity

800

y = 4.6614x - 0.0887 R² = 0.9452 -1 K = 4.5×103 M

0.6

0.015

ip t

1000

Wavelength (nm)

Wavelength (nm)

Fig. 7. The variation in fluorescence emission spectra of L1 (a) and L2 (b) in the presence of

659

increasing concentrations of Cr3+ (0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100

660

µM). Inset: Linear regression plot of fluorescence intensity change 1/(I-I0) as a function of

661

concentration 1/[Cr3+] (top), fluorescence enhancement change as a function of concentration

662

of Cr(III) added (bottom).

M

an

us

658

666 667 668 669 670 671

te

665

Ac ce p

664

d

663

672

32

Page 32 of 49

700

L1+Cr L2+Cr

3+

500 400 300

ip t

Fluorescence Intensity

600

3+

200

0

0.2

0.4

673

0.6

Mole fraction of Cr

0.8

3+

1.0

us

0.0

cr

100

Fig. 8. Job’s plot for L1 and L2 with Cr3+, fluorescence intensity at 575 nm was plotted as a

675

function of the molar ratio.

an

674

676

M

677

681 682 683 684 685 686

te

680

Ac ce p

679

d

678

33

Page 33 of 49

ip t cr us

687

Fig. 9. ESI-MS spectrum of L1 (a) and L2 (b) upon addition of CrCl3.6H2O (1.0 equiv.) in

689

MeOH.

an

688

M

690

694 695 696 697 698 699

te

693

Ac ce p

692

d

691

700 701 702 703 34

Page 34 of 49

500

a

n+

3+

n+

b

L1+M +Cr

L1+M

n+

200

0

cr

Fe3+

Gd3+

Fe2+

Cu2+

Co2+

none

Cd2+

Zn2+

Ni2+

Pb2+

Nd3+

Hg2+

Mn2+

Gd3+

Fe3+

Fe2+

Cu2+

Co2+

Cd2+

none

0

Zn2+

100

Pb2+

200

ip t

300

Ni2+

400

300

Nd3+

500

Hg2+

600

100

704

3+

n+

L2+M +Cr

L2+M

400

Fluorescence Intensity

Fluorescence Intensity

700

Mn2+

800

Fig. 10. Competitive selectivity of probes L1 (a) and L2 (b) toward various metal ions (1.0

706

equiv.) in the absence (black bars) and presence (red bars) of Cr3+ (1.0 equiv.) with an

707

excitation of 520 nm.

an

us

705

708

M

709

713 714 715 716 717 718

te

712

Ac ce p

711

d

710

719 720 721

35

Page 35 of 49

2.0

2.0

b

a 1.6

1.08

0.8

0.81

0.54

0.27

0.00

2

4

6

0.4

8

10

pH

1.25

1.00

0.8

0.75

0.50

0.25

0.00

0.4

2

6

pH

450

722

500

550

600

310

650

372

434

650

c

d

us

516

Fluorescence Intensity

520

430 400

pH2 pH3 pH4 pH5 pH6 pH7 pH8 pH9 pH10

100

0

4

6

8

10

260

130

pH

0

pH2 pH3 pH4 pH5 pH6 pH7 pH8 pH9 pH10

an

86

200

2

390

M

172

300

Fluorescence Intensity

258

496

558

620

Wavelength (nm)

Wavelength (nm)

344

10

cr

400

8

400

Fluorescence Intensity

350

Fluorescence Intensity

4

0.0

0.0

pH2 pH3 pH4 pH5 pH6 pH7 pH8 pH9 pH10

1.2

ip t

pH2 pH3 pH4 pH5 pH6 pH7 pH8 pH9 pH10

1.35

Absorbance

Absorbance

1.62

1.2

Absorbance

Absorbance

1.6

300

200

100

0

2

4

6

8

10

pH

0

533

546

559

723

572

585

598

611

624

650

533

546

559

572

585

598

611

624

637

650

Wavelength (nm)

d

Wavelength (nm)

637

Fig. 11. UV-vis absorbance spectral changes of L1 (a) and L2 (b), Fluorescence emission

725

intensities (both excitation and emission slit widths were 1.5 nm) of L1 (c) and L2 (d) with

726

Cr3+ as a function of pH. Inset: Color changes of L1+Cr3+ and L2+Cr3+ in different pH media

727

under a normal (a, b) and fluorescent (c, d) light (top), absorbance (a and b, at 555 nm) and

728

emission (c and d, at 575 nm) intensities of L1 and L2 in the presence of Cr3+ with pH

729

variation (bottom).

731

Ac ce p

730

te

724

732 733 734 735 36

Page 36 of 49

800

a

700 600 500

0

3+

Cr

Cr

3+

0

600

300

60

80

100

[EDTA]/µM

400

200

100 µM 0

200

40

L1

EDTA

0

1

EDTA

EDTA

EDTA

2

3

4

Cycles

3+

Cr

132

88

3+

0

Cr 200

20

40

50

0

527

544

561

578

595

612

629

100

100 µM

L2

EDTA

0

1

EDTA

EDTA

EDTA

2

3

4

EDTA

0.0 µM

0

510

80

100

44

0.0 µM

60

[EDTA]/µM

150

Cycles

0

736

50

3+

Cr

EDTA

100

100

0

3+

Cr

250

176

150

646

510

Wavelength (nm)

527

544

561

578

595

612

629

646

cr

400

20

220

0µm 5µm 10µm 15µm 20µm 25µm 30µm 35µm 40µm 45µm 50µm 60µm 70µm 80µm 90µm 100µm

ip t

3+

Cr

Fluorescence Intensity

264 200

Fluorescence Intensity at 575 nm

Cr3+

200

300

100

500

250

b

400

Fluorescence Intensity

Fluorescence Intensity

600

Fluorescence Intensity at 575 nm

Fluorescence Intensity

700

Wavelength (nm)

Fig. 12. The variation in fluorescence emission spectra of L1+Cr3+ (a) and L2+Cr3+ (b) upon

738

addition of EDTA (0, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 µM). Inset: Color

739

changes of probe+Cr3+ upon addition of EDTA (2 equiv.) (Top left), fluorescence spectral

740

changes at 575 nm as a function of the amount of EDTA (right) and recovery of the

741

molecular fluorescence at 575 nm of the sensor (50 µM) after addition of Cr3+ (1 equiv.) after

742

each addition of 2 equiv. of EDTA (Bottom left).

745 746 747 748

an

M

d te

744

Ac ce p

743

us

737

749 750 751 37

Page 37 of 49

O R

N

N

Cr3+

O

Cr3+

N

N

R

EDTA N

O

N

turn-OFF

N

turn-ON OH

OH L2 : R =

752

cr

L1 : R =

O

ip t

N

OH

Scheme 2. Proposed binding mechanism of Cr(III) with probes in the presence and absence

754

of EDTA.

an

us

753

755

M

756 757

761 762 763 764 765 766

te

760

Ac ce p

759

d

758

767 768 769 770 38

Page 38 of 49

0.5

L1 L2

0.3

Optical Band Gap

0.2

Corrected Baseline

0.0 360

380

771

775

781 782 783

te Ac ce p

780

d

776

779

460

M

774

778

440

Fig. 13. Absorption spectra and optical band gaps of probes L1 and L2.

773

777

420

an

772

400

Wavelength (nm)

us

340

cr

0.1

ip t

Absorbance

0.4

784 785 786 787 39

Page 39 of 49

1.10

L2+Cr3+ (abs) L2+Cr3+ (em)

0.88

0.66

0.44

0.44

0.22

0.22

ip t

0.66

0.00

cr

Normalized Absorbance

0.88

Normalized Fluorescence Intensity

1.10

L1+Cr3+ (abs) L1+Cr3+ (em)

0.00

480

528

788

576

624

us

Wavelength (nm)

Fig. 14. UV-vis absorption and fluorescence emission spectra of ligands and the

790

corresponding Cr3+ addition products in MeOH−DMSO (99:1 v/v).

an

789

791

M

792

796 797 798 799 800 801

te

795

Ac ce p

794

d

793

802 803 804 805 40

Page 40 of 49

0.0

0.0

a

L1 + Cr

0.5

3+

1.0

2.0

Current/µA

1.5

Eox= 0.572 E = 0.552 ox

1.5

Ferrocene

2.0

2.5

2.5

3.0

3.0

3.5

3.5

Eox= 0.596 Eox= 0.556

4.0 1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

cr

4.0

ip t

Ferrocene

Current/µA

3+

L2 + Cr

0.5

1.0

806

L2

b

L1

0.0

-0.2

Potential/V

Potential/V

Fig. 15. Differential pulse voltammograms recorded for L1 (a) and L2 (b), and the

808

corresponding Cr3+ addition products in MeOH−DMSO (99:1 v/v).

us

807

an

809 810

M

811

815 816 817 818 819 820

te

814

Ac ce p

813

d

812

821 822 823 824 41

Page 41 of 49

ip t us

cr 826

an

825

Fig. 16. Energy level diagram of the probes and the corresponding Cr3+ addition products.

M

827

831 832 833 834 835 836

te

830

Ac ce p

829

d

828

837 838 839 840 42

Page 42 of 49

ip t cr us an M 843 844 845 846

te

d 842

Ac ce p

841

Fig. 17. Optimized structures of L1 and L2 and the corresponding Cr3+ addition products.

847 848 849 850 43

Page 43 of 49

ip t cr us

851

Fig. 18. HOMO and LUMO orbitals of probes (L1 and L2) and its corresponding Cr3+

853

complexes.

an

852

M

854

858 859 860 861 862 863

te

857

Ac ce p

856

d

855

864 865 866 867 44

Page 44 of 49

ip t cr us an M

868

871 872 873 874 875 876

d

te

870

Fig. 19. 1H NMR titration of L1 with Cr3+ in DMSO-d6+CD3OD.

Ac ce p

869

877 878 879 880 45

Page 45 of 49

881

Fig. 20. Fluorescence image of PVC polymeric thin film doped with ligand. The polymeric

883

film on the glass slide was irradiated with a UV lamp.

ip t

882

884

cr

885

us

886 887

an

888 889

M

890

894 895 896 897 898 899

te

893

Ac ce p

892

d

891

900 901 902 903 46

Page 46 of 49

904

Table 1. Absorbance and emission enhancement factor, and corresponding quantum yields of

905

L1 and L2 in the presence of Cr(III) Emission

Quantum

EF (A/Ao*)

EF (I/Io*)

yield (Ф)

263

0.47

L1+Cr3+

4.0

500

261

0.44

L1

6.5

1

1

< 0.001

L1+Cr3+

6.5

103

114

0.14

L1

> 7.0

~1

~1

< 0.001

L1+Cr3+

> 7.0

~1

~1

< 0.001

L2

4.0

397

286

0.51

L2+Cr3+

4.0

402

285

0.57

L2

6.5

1

1

< 0.001

L2+Cr3+

6.5

18

38

L2

> 7.0

~1

~1

L2+Cr3+

> 7.0

~1

~1

ip t

497

cr

4.0

M

an

L1

0.1

d

< 0.001 < 0.001

te

* Ao = Absorbance of probe at neutral pH at 555 nm,

Io = Emission intensity of probe at neutral pH at 575 nm

Ac ce p

906 907 908

Absorbance pH

us

System

47

Page 47 of 49

Vitae

ip t

908

cr

909

Vinod Kumar Gupta obtained his PhD degree in chemistry from the University of Roorkee

911

(now Indian Institute of Technology Roorkee) Roorkee, India, in 1979. Since then he is

912

pursuing research at the same Institute and presently holding the position of Professor,

913

Chemistry Department, at Indian Institute of Technology Roorkee, Roorkee. He worked as a

914

post-doctoral fellow at University of Regensburg, Germany, in 1993 as an EC fellow. He has

915

published more than 400 research papers, many reviews and two books which fetched him

916

more than 31,500 citations with h-index of 121. He was awarded the Indian Citation Laureate

917

Award in 2004 and elected Fellow of the National Academy of Sciences (FNASc) in the year

918

2008.

an

M

d

te

Ac ce p

919

us

910

920

Ashok Kumar Singh is in teaching and research profession for almost 30 years. Presently,

921

he is a Professor of Chemistry at Indian Institute of Technology Roorkee, India and has more

922

than 150 research publications to his credit. Prof. Singh works extensively in the field of

923

macrocyclic chemistry. 48

Page 48 of 49

ip t

924

Naveen Mergu finished his MSc in 2010 in chemistry at National Institute of Technology

926

Warangal (NITW), India. He is currently pursuing PhD in the area o f chemical sensors under

927

the supervision of V.K. Gupta at Indian Institute of Technology Roorkee (IITR).

us

cr

925

an

928

Ac ce p

te

d

M

929

49

Page 49 of 49