On/Off Fluorescent Chemosensor for Selective Detection of Divalent Iron and Copper Ions: Molecular Logic Operation and Protein Binding.

Here, naphthalene diamine-based β-diketone derivative (compound LH) was successfully used as a dual signaling probe for divalent cations, Fe2+ and Cu2+ ions, in bimodal methods (colorimetric and fluorometric). It showed fluorescent enhancement for Fe2+ ion by photoinduced electron transfer mechanism and fluorescence quenching for Cu2+ ion by charge-transfer process. Binding stoichiometry for [LH-(Fe2+)2] and [LH-(Cu2+)2] was found to be 1:2 by Job's plot method and, the binding constants were calculated as 1.6638 × 1010 and 9.22929 × 108 M-1, respectively. Compound LH exhibited OR and XOR logic gate behavior with H+, Fe2+, and Cu2+ as inputs. Further, the compound LH and bovine serum albumin binding interaction showed quenching of fluorescence by Förster resonance energy-transfer mechanism.

Introduction

Chemosensors have gained much attention due to their recognition of heavy-metal ions and importance in the environmental and biological concentration.[1,2] Chemosensors are primarily attractive due to local observation, sensitive ion-induced fluorescence changes, and real-time examination of the metal-ion content. The metal binding eventually causes a change in fluorescence intensity. Sensors mostly present are linked to a fluorophore and metal-chelating site.[3] The development of sensitive chromogenic probes has been receiving much attention in recent years due to their potential application in clinics, biochemistry, and environment. So far, many chromogenic chemosensors have been developed for selective recognition of different species due to their high sensitivity, selectivity, and simplicity.[4] Among metal ions, copper (Cu2+) and iron (Fe2+) are two of the most important transition-metal ions found in both humans and animals. The drinking water standards and health advisories amounts of copper and iron are limited to 1.0 and 0.3 mg L–1, as revised by the U.S. Environmental Protection Agency (EPA).[5,6] Iron is one of the most important elements for metabolic processes (hemoglobin, myoglobin, and a key element in heme enzymes), being indispensable for plants and animals, and therefore it is extensively distributed in environmental and biological materials.[7,8] Overdosage of iron is toxic to the heart and liver and therefore causes neuroinflammation and Alzheimer’s disease.[9,10] Copper is one of the most significant trace-metal nutrients in our body. It plays a vital role in various processes in organisms and human health. Excessive copper is highly toxic and may cause Wilson’s disease in humans.[11,12]

Recently, a field of intensive research to design bi- and multifunctional fluorescent ion probes has emerged.[13,14] The metal-ion detection for biological samples is highly dependent on pH and environment and can be performed at low concentration, and the fluorescence responses depend on several mechanisms.[15−17] Developing low-cost dual sensors with high stability and sensitivity represents another challenge in the current research, i.e., to develop a dual-mode optical response, a probe that can selectively detect several analytes. In this endeavor, we have developed a dyad system (keto-amine of (Z)-5-(5-((Z)-4-oxopent-2-en-2-ylamino) naphthalen-1-ylamino) pent-3-en-2-one) LH, which contains bis-bidentate N, O sites. This molecule acts as an “ON/OFF” fluorescent chemosensor for divalent iron and copper ions. We explored the molecular logic gate behavior of compound LH with H+, Fe2+, and Cu2+ ions as inputs. Further, to understand the binding capability of compound LH to biomolecular carriers, bovine serum albumin (BSA) protein binding study was also explored.

Results and Discussion

Synthesis

The naphthalene diamine β-diketone derivative of LH, the keto-amine of ((Z)-5-(5-((Z)-4-oxopent-2-en-2-ylamino) naphthalen-1-ylamino) pent-3-en-2-one), was synthesized by Schiff base condensation reaction[18,19] between naphthalene-1,5-diamine and acetylacetone in ethanol, as illustrated below (Scheme ).

Scheme 1

Synthesis of LH and its Molecular Structure

UV–Visible and Fluorescence Spectral Studies

The spectroscopic properties of LH have been investigated by UV–vis absorption spectra and fluorescence emission spectra in H2O/dimethylformamide (DMF) medium. Compound LH showed maximum absorption wavelength at 330 nm. When treated with various cations like Zn2+, Na+, Pb2+, Ni2+, Cu2+, Fe2+, Cd2+, Ca2+, Mn2+, Mg2+, and Al3+, compound LH showed a hyperchromic shift centered at 330 nm for Fe2+ and Cu2+ ions. This shows that compound LH is selective toward Fe2+ and Cu2+ ions (Figure a,b).

Figure 1

(a) Absorbance spectra of compound LH with various metal ions. (b) Fluorescence spectra of compound LH with various metal ions in water/DMF (9.9/0.1).

Fluorescent responses of compound LH showed that when excited at 330 nm, LH exhibited weak fluorescence emission upon addition of various metal cations. However, on the addition of Fe2+ ion, compound LH produced strong fluorescence emission at 410 nm. Moreover, the weak fluorescence of compound LH was further quenched by adding Cu2+ ion, owing to the paramagnetic nature of the latter.[20] Hence, compound LH acts as a dual sensor for Fe2+ with fluorescence enhancement and for Cu2+ with fluorescence quenching. Visual color change of compound LH upon addition of various metal ions under long-wavelength UV–visible light (365 nm) was observed. Compound LH showed a light green fluorescence upon the addition of Fe2+ ion. Addition of other metal ions did not produce any significant color change under UV–visible light.[21,22]

To confirm the selectivity of compound LH toward Fe2+ and Cu2+ ions, competitive experiments were performed with a wide range of metal ions. The resulting fluorescence intensities are illustrated in Figure a,b. As shown in Figure a, the competitive cation spectral changes did not lead to any significant variation in the Fe2+ ion spectra and resulted in similar fluorescence spectra changes in the presence of other metal ions. The data clearly suggest that there is no interference of other metal ions for sensing of Fe2+ ions. Similarly, the fluorescence quenching caused by the Cu2+ ion with most other cations was similar to that caused by Cu2+ alone, as seen in Figure b, with not much variation observed in the intensity levels. These results indicate that the presence of other cations does not interfere significantly with the binding of LH toward Fe2+ and Cu2+ ions.

Figure 2

(a) Fluorescence response of compound LH toward Fe2+ in the presence of various metal ions. (b) Fluorescence response of compound LH toward Cu2+ in the presence of various metal ions.

Binding Interaction

To understand the binding stoichiometry between compound LH and Fe2+/Cu2+, Job’s plot experiment was carried out. Various mole fractions of metal ions, Fe2+/Cu2+, viz., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0, were prepared and their fluorescence intensities were measured. The concentrations of these solutions range from 1 × 10–5 to 1 × 10–4 M. A maximum fluorescence emission intensity was identified at 410 nm when the mole fraction of Fe2+ reached 0.6, which is indicative of 1:2 stoichiometric complexation between compound LH and Fe2+ (Figure a). Similarly, for Cu2+, the minimum emission intensity was at 0.6 mol fraction, which indicates the formation of 1:2 complex (Figure b). The association constants on the basis of the Benesi–Hildebrand equation[23,24] (see Figure S12 in the Supporting Information) and Stern–Volmer quenching equation were calculated as 1.6638 × 1010 and 9.22929 × 108 M–1 for [LH–(Fe2+)2] and [LH–(Cu2+)2], respectively (see Figure S13 in the Supporting Information). The detection limits were calculated on the basis of the fluorescence titrations to be 5 × 10–7 M for [LH–(Fe2+)2] and 1 × 10–7 M for [LH–(Cu2+)2] (see Figure S10 in the Supporting Information).

Figure 3

Job’s plots indicating (a) 1:2 stoichiometry for [LH–(Fe2+)2] complex and (b) 1:2 stoichiometry for [LH–(Cu2+)2] complex.

To further understand the interaction between compound LH and Fe2+/Cu2+ ions, 1H NMR titration experiments for Fe2+ and Cu2+ were carried out in dimethyl sulfoxide. The 1H NMR peak of imine proton of compound LH is found at 12.95 ppm. Upon complexation with Fe2+ ions, the imine proton is shifted downfield to 14.2 ppm. Similarly, on the addition of Cu2+ to LH, the N–H peak was broadened and shifted downfield to 15.65 ppm. These results clearly showed that the coordination takes place in the imine nitrogen and the carbonyl group of compound LH (Figure ). The compound LH was found to be reversible with ethylenediaminetetraacetic acid (EDTA) solution. On adding Fe2+ ion, the fluorescence emission of compound LH was enhanced. But when EDTA was treated with LH–Fe2+ complex, the fluorescence became weak since EDTA is bound to Fe2+ ion leaving the chemosensor free. The same reversibility experiment was conducted with copper ion. This showed that the compound LH is reversible with EDTA (see Figure S5 in the Supporting Information).

Figure 4

1H NMR spectra of compound LH with Fe2+ and Cu2+ cations.

Binding Mechanism for Sensing of Fe2+ and Cu2+ Ions

The proposed binding mode between compound LH and Fe2+/Cu2+ is shown in Scheme . The nonbonding electron pair of nitrogen atom transferred to naphthyl ring results in weak fluorescence emission for LH due to photoinduced electron transfer (PET) process.[25,26]

Scheme 2

Proposed Binding Mode of Compound LH with Fe2+ and Cu2+ Ions

After coordination with Fe2+, the photoinduced electron transfer (PET) from the receptor (N atom) to the fluorophore (naphthyl rings) is blocked, resulting in the switching “ON” of the fluorescence. (Scheme ) Similarly, Cu2+ is a paramagnetic cation, which induces fluorescence quenching response.

The fluorophore opens a nonradiative deactivation channel and facilitates the transfer of electron or energy, resulting in the fluorescence quenching response of (LH + Cu2+). The weak fluorescence of compound LH was completely quenched, which could be ascribed to the metal–ligand charge transfer (MLCT) mechanism.[27] In the complex (LH + Cu2+), paramagnetism could induce rapid occurrence of the MLCT, and the paramagnetic quenching property of Cu2+ was much stronger to cover the other possible mechanisms. Similarly, for the lighted new complex (LH + Cu2+), the paramagnetic quenching property of Cu2+ played the leading role, so (LH + Cu2+) showed a completely quenched fluorescence, i.e., switch “OFF”.

Molecular Logic Gates

OR and XOR logic functions (Figure a,b) with Fe2+ and Cu2+ ions as inputs (Tables and 2) (Scheme ) for two different pH values, 3.0 (acidic state) and 7 (neutral state), were selected.[28]

Figure 5

(a) Fluorescence response (λex = 330 nm) of compound LH (1 × 10–5 M) in the presence and absence of 1 equiv Fe2+ ion. (b) Fluorescence response of compound LH (1 × 10–5 M) in the presence and absence of 1 equiv Cu2+ ion in H2O/DMF (9:1 v/v) at pHs 3.0 and 7.

Table 1

Truth Table of OR Logic Gate

threshold	input [H+]	input [Cu2+]	output (410 nm)
compound (LH) [1000]	0	0	0
	1	0	1
	0	1	1
	1	1	0

Table 2

Truth Table of Logic Gate

threshold	input [H+]	input [Fe2+]	output (410 nm)
compound (LH) [1000]	0	0	0
	1	0	1
	0	1	1
	1	1	1

Scheme 3

OR and XOR Logic Gates

In neutral condition, the fluorescence intensity of compound LH enhanced at 410 nm after adding 1 equiv Fe2+ ion. In acidic medium, the addition of Fe2+ ion to compound LH exhibited fluorescence enhancement compared to the neutral condition of compound LH, along with a red shift. In neutral medium, the fluorescence intensity of compound LH after adding 1 equiv Cu2+ showed a blue shift with fluorescence quenching at 410 nm. However, in acidic condition, compound LH with the addition of Cu2+ exhibited fluorescence enhancement accompanied by a blue shift (see Figures S7 and S8 in the Supporting Information).

Fluorescence Imaging of Compound LH with Fe2+

The ability of compound LH to detect Fe2+ ion was examined by fluorescence imaging. The chemosensor LH was loaded with 10 μL of Fe2+ at 30 °C for laser scanning confocal microscopy. The fluorescence images grew brighter with an increase in the concentration of Fe2+ ion (Figure ). These images proved that a strong fluorescence enhancement resulted when Fe2+ was added to compound LH. These results support that compound LH is an effective fluorescent sensor for Fe2+ metal ion.

Figure 6

Fluorescence image of compound LH with Fe2+ metal ion.

Binding of Compound LH to BSA

The binding titration of compound LH against BSA is shown in Figure a. The absorbance spectrum of BSA showed two absorbance bands, viz., 280 and 330 nm. The increasing concentration of compound LH led to a hyperchromic shift with the formation of a new absorbance band at 330 nm (see Figure S14 in the Supporting Information). The fluorescence spectra for the binding of BSA to compound LH were recorded. As shown in Figure b, fluorescence quenching was observed for the protein by increasing the concentration of compound LH.

Figure 7

(a) Absorbance and (b) fluorescence spectra showing the binding titration of compound LH to BSA.

The Stern–Volmer quenching plot for the interaction of protein with compound LH was studied. The binding constant (K) of compound LH to BSA was calculated as 4.96 × 10–2 M–1 (correlation coefficient = 0.99) from Figure .

Figure 8

(a) Fluorescence spectra of compound LH with various concentrations. (b) Stern–Volmer quenching plot for the binding of BSA to compound LH.

Förster Resonance Energy Transfer

The overlapping of the absorption spectra of the acceptor molecule with the fluorescence emission spectra of the donor molecule is the indication of compound LH being the acceptor molecule and BSA being the donor molecule.

The overlap integral is given byFrom the spectral overlap of the fluorescence emission spectrum of BSA and the absorption spectrum of compound LH (Figure ), the overlap integral J is calculated to be 5.9322 × 10–20 cm3 mol–1 dm3. The K2 value here is 2/3 and then Φ = 0.15, n = 1.33, and E = 0.06389. The calculated values of R0 and r are 4.6844 and 7.3275 nm, respectively, which suggests that the energy transfer from BSA to compound LH occurs with a good probability.[29,30]

Figure 9

Spectral overlapping between the UV–visible spectrum of compound LH and the fluorescence spectrum of BSA.

Conclusions

We have developed an ON/OFF dual fluorescent chemosensor based on a naphthalene diamine β-diketone derivative LH for the selective detection of Fe2+ and Cu2+ ions. The fluorescence emission intensity of LH was remarkably high after the addition of Fe2+ ion, and quenching was observed after the addition of Cu2+ ion. The compound LH binds to both the metal ions in a 1:2 stoichiometry. Binding constants were calculated as 1.6638 × 1010 and 9.22929 × 108 M–1 for [LH–(Fe2+)2] and [LH–(Cu2+)2], respectively. Further, LH exhibited OR and XOR logic gate behavior with H+, Fe2+, and Cu2+ as inputs. Binding interaction between bovine serum albumin and compound LH was observed by fluorescence quenching and Förster resonance energy transfer. On the basis of the results obtained, we suggest that the compound LH can be used as a dual sensor for simultaneous detection of Fe2+ and Cu2+ metal ions.

Experimental Section

Materials and Instrumentation

All reagents and chemicals, unless stated otherwise, were purchased from Sigma-Aldrich. Naphthalene-1,5-diamine and acetylacetone were purchased from Aldrich. The solvents, viz., petroleum ether, hexane, chloroform, methanol, dimethyl sulfoxide, tetrahydrofuran, and ethyl acetate, were obtained from Avra. Ethanol (99%), used as a solvent in the synthesis, was bought from Aldrich. All experiments were conducted using double-distilled water.

The fluorescence emission spectra were collected on a Jasco FP-8300 spectrofluorometer, and the UV–vis spectra were recorded on a double-beam Jasco V-630 spectrophotometer with excitation and emission slits at 5.0 nm. The pH was measured using an Elico LI 120 pH meter (India), and fluorescence microscopy images were recorded using a Nikon ECLIPSE TS100 laser scanning confocal microscope. 1H NMR spectra were recorded in a CDCl3 solvent on a Bruker Varian Inova 300 MHz FT-NMR spectrometer. The chemical shifts for proton resonances are reported in ppm (δ) relative to tetramethylsilane.

Synthesis of Compound LH

Compound LH (keto-amine of (Z)-5-(5-((Z)-4-oxopent-2-en-2-ylamino) naphthalen-1-ylamino) pent-3-en-2-one) was synthesized through simple condensation reaction between naphthalene-1,5-diamine and acetylacetone. Ethanolic solution of naphthalene-1,5-diamine (0.001 mol) and acetylacetone (0.002 mol) was mixed, and few drops of acetic acid was added to the mixture with continuous stirring. The solution was refluxed for 3 h and cooled to room temperature (RT). The mixture was slowly evaporated in vacuum at RT for 48 h to afford yellowish brown crystals. Keto-amine of ((Z)-5-(5-((Z)-4-oxopent-2-en-2-ylamino)naphthalen-1-ylamino) pent-3-en-2-one): 1H NMR (CDCl3, 300 MHz, δ, ppm): 12.78 (broad s, NH, 2H), 7.98, 7.95 (d, J = 10 Hz, Ar-CH, 2H), 7.53, 7.51, 7.48 (t, J = 10 Hz, 5 Hz, Ar-CH, 2H), 7.34, 7.31 (d, J = 5 Hz, Ar-CH, 2H), 5.32 (s, =CH, 2H), 2.18 (s, CH3, 6H), 1.89 ppm (s, CH3, 6H). 13C NMR: (δ, CDCl3) 194.52 ppm (C=O), 159.38 ppm (NH–CH2), 111–141 ppm (aromatic) 95 ppm (−CH2–C=O), 17–27 ppm (Me). FT-IR: (KBr, υ, cm–1) 1619 (m, C=O stretch), 1606 (m, C=C stretch), 1280 (NH bend stretch), 1427 (C–C in ring stretch), 1354 (C–H stretch). ESI-MS m/z [M + 1]+ calculated for C20H22N2O2: 322.40, found 323.5. UV–vis: DMF, λmax: 330 nm.

Preparation of Solutions

UV–Visible and Fluorescence Titration

The stock solution of compound LH (1 × 10–3 M) was prepared by dissolving 0.0032 g of it in 10 mL of DMF. Various metal salt solutions, viz., 1 × 10–3 M MgSO4, FeSO4, CaSO4, Na2SO4, CuSO4, MnSO4, Pb(NO3)2, CdSO4, NiSO4, ZnSO4, and Al2(SO4)3, were prepared by dissolving them in distilled water. LH (100 μL) and 100 μL of different metal-ion solution were diluted in 10 mL of double-distilled water to attain the final concentration of 1 × 10–5 mol dm–3. After shaking the standard measuring flask for a few minutes, the UV–visible and fluorescence spectra were recorded at room temperature.

Competitive Binding Studies

The stock solution of compound LH (0.001 M) in DMF (10 mL) was prepared. LH solution (100 μL), 100 μL of different metal-ion solution (0.001 M), and 100 μL of sensed metal-ion solution (0.001 M) were diluted to 10 mL of water (pH 7.0).

Solutions of Various pHs

Various pH solutions were prepared by mixing orthophosphoric acid (1 × 10–1 mol dm–3) and NaOH (1 × 10–3 mol dm–3). Compound LH (100 μL) in DMF was mixed with 100 μL of iron or copper metal-ion solution and diluted to 10 mL of various pH solutions.

Preparation of Test Solutions of BSA for Binding of Compound LH

N-(2-Hydroxyethyl)piperazine-N′-ethanesulfonic acid buffer (0.1 M) was used for the preparation of a stock solution of BSA (3.0 × 10–5 mol dm–3). The concentration of DMF used was 1%. The titration of compound LH with BSA was carried out at different concentrations of LH, viz., 0, 1 × 10–6, 2 × 10–6, 4 × 10–6, 6 × 10–6, 8 × 10–6, 1 × 10–5, and 2 × 10–5 mol dm–3, to BSA. The experiments were carried out at an ambient temperature of 27 ± 2 °C. The fluorescence and absorption spectra were recorded against appropriate compound LH solution.